Method for supporting carrier sensing in wireless access system supporting unlicensed band, and device supporting same

ABSTRACT

The present invention relates to methods for supporting carrier sensing in a wireless access system supporting an unlicensed band, and devices supporting same. A method for supporting carrier sensing in a wireless access system supporting an unlicensed band, according to an embodiment of the present invention, comprises the steps of: selecting a backoff counter value for performing carrier sensing; performing carrier sensing in a serving cell (S-cell) until the backoff counter value reaches a predetermined threshold value; when the backoff counter value reaches the predetermined threshold value, transmitting data in a transmission opportunity (TXOP) interval; receiving a reception state feedback indicating whether the transmitting of data is success, collision or error; and resetting the backoff counter value on the basis of the reception state feedback, wherein the S-cell can be configured in an unlicensed band.

This application is a National Phase Application of International Application No. PCT/KR2015/006693, filed on Jun. 30, 2015, which claims the benefit of U.S. Provisional Application No. 62/021,681, filed Jul. 7, 2014, which are hereby incorporated by reference in their entirety for all purposes as if fully set forth herein.

TECHNICAL FIELD

The present invention relates to a wireless access system supporting an unlicensed band, and more particularly, to methods of supporting carrier sensing for determining whether a serving cell of an unlicensed band is in an idle state, methods of transmitting and receiving data in the serving cell, and apparatuses therefor.

BACKGROUND ART

Wireless access systems have been widely deployed to provide various types of communication services such as voice or data. In general, a wireless access system is a multiple access system that supports communication of multiple users by sharing available system resources (a bandwidth, transmission power, etc.) among them. For example, multiple access systems include a Code Division Multiple Access (CDMA) system, a Frequency Division Multiple Access (FDMA) system, a Time Division Multiple Access (TDMA) system, an Orthogonal Frequency Division Multiple Access (OFDMA) system, and a Single Carrier Frequency Division Multiple Access (SC-FDMA) system.

DISCLOSURE OF THE INVENTION Technical Tasks

An object of the present invention is to provide a method of transmitting and receiving reliable data in a wireless access system supporting an unlicensed band.

Another object of the present invention is to provide various carrier sensing methods for checking whether a serving cell is in an idle state or a busy state in a wireless access system supporting an unlicensed band.

Another object of the present invention is to provide methods of supporting carrier sensing in a wireless access system supporting an unlicensed band.

Another object of the present invention is to provide methods of configuring a unit in which carrier sensing is performed.

The other object of the present invention is to provide apparatuses supporting the aforementioned methods.

It will be appreciated by persons skilled in the art that the objects that could be achieved with the present invention are not limited to what has been particularly described hereinabove and the above and other objects that the present invention could achieve will be more clearly understood from the following detailed description.

Technical Solution

The present invention relates to methods of supporting carrier sensing in a wireless access system supporting an unlicensed band and devices supporting the same.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, according to one embodiment, a method of supporting carrier sensing in a wireless access system supporting an unlicensed band, includes the steps of selecting a backoff counter value for performing carrier sensing, performing the carrier sensing in a serving cell (S-cell) until the backoff counter value reaches a prescribed threshold value, if the backoff counter value reaches the prescribed threshold value, transmitting data in a transmission opportunity (TXOP) section, receiving a reception state feedback indicating whether transmission of the data corresponds to a success, a collision, or an error, and reconfiguring the backoff counter value based on the reception state feedback. In this case, S-cell can be configured in the unlicensed band.

To further achieve these and other advantages and in accordance with the purpose of the present invention, according to a different embodiment, a device supporting carrier sensing in a wireless access system supporting an unlicensed band can include a transmitter, a receiver, and a processor configured to control the transmitter and the receiver to support the carrier sensing.

In this case, the processor is configured to select a backoff counter value for performing the carrier sensing, configured to control the receiver to perform the carrier sensing in an S-cell until the backoff counter value reaches a prescribed threshold value, if the backoff counter value reaches the prescribed threshold value, configured to control the transmitter to transmit data in a transmission opportunity (TXOP) section, configured to control the receiver to receive a reception state feedback indicating whether transmission of the data corresponds to a success, a collision, or an error, and configured to reconfigure the backoff counter value based on the reception state feedback. In this case, the S-cell can be configured in the unlicensed band.

In the embodiments of the present invention, the data is transmitted through the S-cell, the reception state feedback is transmitted through a primary cell (P-cell), and the P-cell may correspond to a serving cell configured in a licensed band.

If the transmission of the data corresponds to the success, the reception state feedback includes an ACK signal, if the transmission of the data corresponds to the error, the reception state feedback includes an NACK signal, and if the transmission of the data corresponds to the collision, the reception state feedback can be configured not to include both the ACK signal and the NACK signal.

The carrier sensing can be performed in a carrier sensing unit. In this case, the carrier sensing unit corresponds to 1 OFDM (orthogonal frequency division multiplexing) symbol unit, half OFDM symbol unit, or 1/N OFDM symbol unit. In this case, the N may correspond to a natural number.

If the carrier sensing is continuously determined as an idle state for the last n number of sections in the N number of 1/N OFDM symbol units, the S-cell can be determined as in an idle state.

It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

Advantageous Effects

According to embodiments of the present invention, it is able to reliably transmit and receive data in a wireless access system supporting an unlicensed band.

And, it is able to stably configure a TXOP duration for transmitting and receiving data by efficiently determining whether a channel state of an S-cell configured in an unlicensed band is in busy or idle state.

And, it is able to efficiently perform carrier sensing by defining a carrier sensing unit.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention.

FIG. 1 is a conceptual diagram illustrating physical channels used in the embodiments and a signal transmission method using the physical channels.

FIG. 2 is a diagram illustrating a structure of a radio frame for use in the embodiments.

FIG. 3 is a diagram illustrating an example of a resource grid of a downlink slot according to the embodiments.

FIG. 4 is a diagram illustrating a structure of an uplink subframe according to the embodiments.

FIG. 5 is a diagram illustrating a structure of a downlink subframe according to the embodiments.

FIG. 6 illustrates PUCCH formats 1a and 1b for use in a normal cyclic prefix (CP) case, and FIG. 7 illustrates PUCCH formats 1a and 1b for use in an extended CP case.

FIG. 8 illustrates PUCCH formats 2/2a/2b in a normal cyclic prefix (CP) case, and FIG. 9 illustrates PUCCH formats 2/2a/2b in an extended CP case.

FIG. 10 illustrates ACK/NACK channelization for PUCCH formats 1a and 1b.

FIG. 11 illustrates channelization for a hybrid structure of PUCCH format 1a/1b and format 2/2a/2b in the same PRB.

FIG. 12 illustrates allocation of a physical resource block (PRB).

FIG. 13 is a diagram illustrating an example of a component carrier (CC) of the embodiments and carrier aggregation (CA) used in an LTE_A system.

FIG. 14 illustrates a subframe structure of an LTE-A system according to cross-carrier scheduling.

FIG. 15 is conceptual diagram illustrating a construction of serving cells according to cross-carrier scheduling.

FIG. 16 is a conceptual diagram illustrating CA PUCCH signal processing.

FIG. 17 is a diagram showing an example of a new PUCCH format based on block spreading.

FIG. 18 is a diagram showing an example of configuring a resource block in time-frequency units.

FIG. 19 is a diagram showing an example of a resource allocation and retransmission method of an asynchronous HARQ method.

FIG. 20 is a diagram for an example of a CA environment supported by LTE-U system.

FIG. 21 is a diagram for explaining methods of performing CS in an S-cell.

FIG. 22 is a diagram for explaining a case that a unit in which CS is performed corresponds to 1/N OFDM symbol unit.

FIG. 23 is a diagram for explaining methods of performing CS in consideration of an unlicensed band.

FIG. 24 is a flowchart for explaining a method of performing CS in consideration of a channel state.

FIG. 25 is a diagram for a device capable of implementing the methods described in FIGS. 1 to 24.

BEST MODE Mode For Invention

The present invention provides methods of supporting carrier sensing in a wireless access system supporting an unlicensed band and apparatuses supporting the methods.

The embodiments of the present disclosure described below are combinations of elements and features of the present disclosure in specific forms. The elements or features may be considered selective unless otherwise mentioned. Each element or feature may be practiced without being combined with other elements or features. Further, an embodiment of the present disclosure may be constructed by combining parts of the elements and/or features. Operation orders described in embodiments of the present disclosure may be rearranged. Some constructions or elements of any one embodiment may be included in another embodiment and may be replaced with corresponding constructions or features of another embodiment.

In the description of the attached drawings, a detailed description of known procedures or steps of the present disclosure will be avoided lest it should obscure the subject matter of the present disclosure. In addition, procedures or steps that could be understood to those skilled in the art will not be described either.

Throughout the specification, when a certain portion “includes” or “comprises” a certain component, this indicates that other components are not excluded and may be further included unless otherwise noted. The terms “unit”, “-or/er” and “module” described in the specification indicate a unit for processing at least one function or operation, which may be implemented by hardware, software or a combination thereof. In addition, the terms “a or an”, “one”, “the” etc. may include a singular representation and a plural representation in the context of the present invention (more particularly, in the context of the following claims) unless indicated otherwise in the specification or unless context clearly indicates otherwise.

In the embodiments of the present disclosure, a description is mainly made of a data transmission and reception relationship between a Base Station (BS) and a User Equipment (UE). A BS refers to a terminal node of a network, which directly communicates with a UE. A specific operation described as being performed by the BS may be performed by an upper node of the BS.

Namely, it is apparent that, in a network comprised of a plurality of network nodes including a BS, various operations performed for communication with a UE may be performed by the BS, or network nodes other than the BS. The term ‘BS’ may be replaced with a fixed station, a Node B, an evolved Node B (eNode B or eNB), an Advanced Base Station (ABS), an access point, etc.

In the embodiments of the present disclosure, the term terminal may be replaced with a UE, a Mobile Station (MS), a Subscriber Station (SS), a Mobile Subscriber Station (MSS), a mobile terminal, an Advanced Mobile Station (AMS), etc.

A transmitter is a fixed and/or mobile node that provides a data service or a voice service and a receiver is a fixed and/or mobile node that receives a data service or a voice service. Therefore, a UE may serve as a transmitter and a BS may serve as a receiver, on an UpLink (UL). Likewise, the UE may serve as a receiver and the BS may serve as a transmitter, on a DownLink (DL).

The embodiments of the present disclosure may be supported by standard specifications disclosed for at least one of wireless access systems including an Institute of Electrical and Electronics Engineers (IEEE) 802.xx system, a 3^(rd) Generation Partnership Project (3GPP) system, a 3GPP Long Term Evolution (LTE) system, and a 3GPP2 system. In particular, the embodiments of the present disclosure may be supported by the standard specifications, 3GPP TS 36.211, 3GPP TS 36.212, 3GPP TS 36.213, 3GPP TS 36.321 and 3GPP TS 36.331. That is, the steps or parts, which are not described to clearly reveal the technical idea of the present disclosure, in the embodiments of the present disclosure may be explained by the above standard specifications. All terms used in the embodiments of the present disclosure may be explained by the standard specifications.

Reference will now be made in detail to the embodiments of the present disclosure with reference to the accompanying drawings. The detailed description, which will be given below with reference to the accompanying drawings, is intended to explain exemplary embodiments of the present disclosure, rather than to show the only embodiments that can be implemented according to the invention.

The following detailed description includes specific terms in order to provide a thorough understanding of the present disclosure. However, it will be apparent to those skilled in the art that the specific terms may be replaced with other terms without departing the technical spirit and scope of the present disclosure.

Hereinafter, 3GPP LTE/LTE-A system which is one of wireless access systems applicable to the embodiments of the present invention will be explained.

The embodiments of the present disclosure can be applied to various wireless access systems such as Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA), etc.

CDMA may be implemented as a radio technology such as Universal Terrestrial Radio Access (UTRA) or CDMA2000. TDMA may be implemented as a radio technology such as Global System for Mobile communications (GSM)/General packet Radio Service (GPRS)/Enhanced Data Rates for GSM Evolution (EDGE). OFDMA may be implemented as a radio technology such as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Evolved UTRA (E-UTRA), etc.

UTRA is a part of Universal Mobile Telecommunications System (UMTS). 3GPP LTE is a part of Evolved UMTS (E-UMTS) using E-UTRA, adopting OFDMA for DL and SC-FDMA for UL. LTE-Advanced (LTE-A) is an evolution of 3GPP LTE. While the embodiments of the present disclosure are described in the context of a 3GPP LTE/LTE-A system in order to clarify the technical features of the present disclosure, the present disclosure is also applicable to an IEEE 802.16e/m system, etc.

1. 3GPP LTE/LTE-A System

In a wireless access system, a UE receives information from an eNB on a DL and transmits information to the eNB on a UL. The information transmitted and received between the UE and the eNB includes general data information and various types of control information. There are many physical channels according to the types/usages of information transmitted and received between the eNB and the UE.

1.1 System Overview

FIG. 1 illustrates physical channels and a general method using the physical channels, which may be used in embodiments of the present disclosure.

When a UE is powered on or enters a new cell, the UE performs initial cell search (S11). The initial cell search involves acquisition of synchronization to an eNB. Specifically, the UE synchronizes its timing to the eNB and acquires information such as a cell Identifier (ID) by receiving a Primary Synchronization Channel (P-SCH) and a Secondary Synchronization Channel (S-SCH) from the eNB.

Then the UE may acquire information broadcast in the cell by receiving a Physical Broadcast Channel (PBCH) from the eNB.

During the initial cell search, the UE may monitor a DL channel state by receiving a Downlink Reference Signal (DL RS).

After the initial cell search, the UE may acquire more detailed system information by receiving a Physical Downlink Control Channel (PDCCH) and receiving a Physical Downlink Shared Channel (PDSCH) based on information of the PDCCH (S12).

To complete connection to the eNB, the UE may perform a random access procedure with the eNB (S13 to S16). In the random access procedure, the UE may transmit a preamble on a Physical Random Access Channel (PRACH) (S13) and may receive a PDCCH and a PDSCH associated with the PDCCH (S14). In the case of contention-based random access, the UE may additionally perform a contention resolution procedure including transmission of an additional PRACH (S15) and reception of a PDCCH signal and a PDSCH signal corresponding to the PDCCH signal (S16).

After the above procedure, the UE may receive a PDCCH and/or a PDSCH from the eNB (S17) and transmit a Physical Uplink Shared Channel (PUSCH) and/or a Physical Uplink Control Channel (PUCCH) to the eNB (S18), in a general UL/DL signal transmission procedure.

Control information that the UE transmits to the eNB is generically called Uplink Control Information (UCI). The UCI includes a Hybrid Automatic Repeat and reQuest Acknowledgement/Negative Acknowledgement (HARQ-ACK/NACK), a Scheduling Request (SR), a Channel Quality Indicator (CQI), a Precoding Matrix Index (PMI), a Rank Indicator (RI), etc.

In the LTE system, UCI is generally transmitted on a PUCCH periodically. However, if control information and traffic data should be transmitted simultaneously, the control information and traffic data may be transmitted on a PUSCH. In addition, the UCI may be transmitted a periodically on the PUSCH, upon receipt of a request/command from a network.

FIG. 2 illustrates exemplary radio frame structures used in embodiments of the present disclosure.

FIG. 2(a) illustrates frame structure type 1. Frame structure type 1 is applicable to both a full Frequency Division Duplex (FDD) system and a half FDD system.

One radio frame is 10 ms (T_(f)=307200·T_(s)) long, including equal-sized 20 slots indexed from 0 to 19. Each slot is 0.5 ms (T_(slot)=15360·T_(s)) long. One subframe includes two successive slots. An i^(th) subframe includes 2i^(th) and (2i+1)^(th) slots. That is, a radio frame includes 10 subframes. A time required for transmitting one subframe is defined as a Transmission Time Interval (TTI). T_(s) is a sampling time given as T_(s)=1/(15 kHz×2048)=3.2552×10⁻⁸ (about 33 ns). One slot includes a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols or SC-FDMA symbols in the time domain by a plurality of Resource Blocks (RBs) in the frequency domain.

A slot includes a plurality of OFDM symbols in the time domain. Since OFDMA is adopted for DL in the 3GPP LTE system, one OFDM symbol represents one symbol period. An OFDM symbol may be called an SC-FDMA symbol or symbol period. An RB is a resource allocation unit including a plurality of contiguous subcarriers in one slot.

In a full FDD system, each of 10 subframes may be used simultaneously for DL transmission and UL transmission during a 10-ms duration. The DL transmission and the UL transmission are distinguished by frequency. On the other hand, a UE cannot perform transmission and reception simultaneously in a half FDD system.

The above radio frame structure is purely exemplary. Thus, the number of subframes in a radio frame, the number of slots in a subframe, and the number of OFDM symbols in a slot may be changed.

FIG. 2(b) illustrates frame structure type 2. Frame structure type 2 is applied to a Time Division Duplex (TDD) system. One radio frame is 10 ms (T_(f)=307200·T_(s)) long, including two half-frames each having a length of 5 ms (=153600·T_(s)) long. Each half-frame includes five subframes each being 1 ms (=30720·T_(s)) long. An i^(th) subframe includes 2i^(th) and (2i+1)^(th) slots each having a length of 0.5 ms (T_(slot)=15360·T_(s)). T_(s) is a sampling time given as T_(s)=1/(15 kHz×2048)=3.2552×10⁻⁸ (about 33 ns).

A type-2 frame includes a special subframe having three fields, Downlink Pilot Time Slot (DwPTS), Guard Period (GP), and Uplink Pilot Time Slot (UpPTS). The DwPTS is used for initial cell search, synchronization, or channel estimation at a UE, and the UpPTS is used for channel estimation and UL transmission synchronization with a UE at an eNB. The GP is used to cancel UL interference between a UL and a DL, caused by the multi-path delay of a DL signal.

[Table 1] below lists special subframe configurations (DwPTS/GP/UpPTS lengths).

TABLE 1 Normal cyclic prefix in downlink Extended cyclic prefix in downlink UpPTS UpPTS Special Normal Extended Normal Extended subframe cyclic prefix cyclic prefix cyclic prefix cyclic prefix configuration DwPTS in uplink in uplink DwPTS in uplink in uplink 0  6592 · T_(s) 2192 · T_(s) 2560 · T_(s)  7680 · T_(s) 2192 · T_(s) 2560 · T_(s) 1 19760 · T_(s) 20480 · T_(s) 2 21952 · T_(s) 23040 · T_(s) 3 24144 · T_(s) 25600 · T_(s) 4 26336 · T_(s)  7680 · T_(s) 4384 · T_(s) 5120 · T_(s) 5  6592 · T_(s) 4384 · T_(s) 5120 · T_(s) 20480 · T_(s) 6 19760 · T_(s) 23040 · T_(s) 7 21952 · T_(s) — — — 8 24144 · T_(s) — — —

FIG. 3 illustrates an exemplary structure of a DL resource grid for the duration of one DL slot, which may be used in embodiments of the present disclosure.

Referring to FIG. 3, a DL slot includes a plurality of OFDM symbols in the time domain. One DL slot includes 7 OFDM symbols in the time domain and an RB includes 12 subcarriers in the frequency domain, to which the present disclosure is not limited.

Each element of the resource grid is referred to as a Resource Element (RE). An RB includes 12×7 REs. The number of RBs in a DL slot, N_(DL) depends on a DL transmission bandwidth. A UL slot may have the same structure as a DL slot.

FIG. 4 illustrates a structure of a UL subframe which may be used in embodiments of the present disclosure.

Referring to FIG. 4, a UL subframe may be divided into a control region and a data region in the frequency domain. A PUCCH carrying UCI is allocated to the control region and a PUSCH carrying user data is allocated to the data region. To maintain a single carrier property, a UE does not transmit a PUCCH and a PUSCH simultaneously. A pair of RBs in a subframe are allocated to a PUCCH for a UE. The RBs of the RB pair occupy different subcarriers in two slots. Thus it is said that the RB pair frequency-hops over a slot boundary.

FIG. 5 illustrates a structure of a DL subframe that may be used in embodiments of the present disclosure.

Referring to FIG. 5, up to three OFDM symbols of a DL subframe, starting from OFDM symbol 0 are used as a control region to which control channels are allocated and the other OFDM symbols of the DL subframe are used as a data region to which a PDSCH is allocated. DL control channels defined for the 3GPP LTE system include a Physical Control Format Indicator Channel (PCFICH), a PDCCH, and a Physical Hybrid ARQ Indicator Channel (PHICH).

The PCFICH is transmitted in the first OFDM symbol of a subframe, carrying information about the number of OFDM symbols used for transmission of control channels (i.e. the size of the control region) in the subframe. The PHICH is a response channel to a UL transmission, delivering an HARQ ACK/NACK signal. Control information carried on the PDCCH is called Downlink Control Information (DCI). The DCI transports UL resource assignment information, DL resource assignment information, or UL Transmission (Tx) power control commands for a UE group.

1.2 Physical Downlink Control Channel (PDCCH)

1.2.1 PDCCH Overview

The PDCCH may deliver information about resource allocation and a transport format for a Downlink Shared Channel (DL-SCH) (i.e. a DL grant), information about resource allocation and a transport format for an Uplink Shared Channel (UL-SCH) (i.e. a UL grant), paging information of a Paging Channel (PCH), system information on the DL-SCH, information about resource allocation for a higher-layer control message such as a random access response transmitted on the PDSCH, a set of Tx power control commands for individual UEs of a UE group, Voice Over Internet Protocol (VoIP) activation indication information, etc.

A plurality of PDCCHs may be transmitted in the control region. A UE may monitor a plurality of PDCCHs. A PDCCH is transmitted in an aggregate of one or more consecutive Control Channel Elements (CCEs). A PDCCH made up of one or more consecutive CCEs may be transmitted in the control region after subblock interleaving. A CCE is a logical allocation unit used to provide a PDCCH at a code rate based on the state of a radio channel A CCE includes a plurality of RE Groups (REGs). The format of a PDCCH and the number of available bits for the PDCCH are determined according to the relationship between the number of CCEs and a code rate provided by the CCEs.

1.2.2 PDCCH Structure

A plurality of PDCCHs for a plurality of UEs may be multiplexed and transmitted in the control region. A PDCCH is made up of an aggregate of one or more consecutive CCEs. A CCE is a unit of 9 REGs each REG including 4 REs. Four Quadrature Phase Shift Keying (QPSK) symbols are mapped to each REG REs occupied by RSs are excluded from REGs. That is, the total number of REGs in an OFDM symbol may be changed depending on the presence or absence of a cell-specific RS. The concept of an REG to which four REs are mapped is also applicable to other DL control channels (e.g. the PCFICH or the PHICH). Let the number of REGs that are not allocated to the PCFICH or the PHICH be denoted by N_(REG). Then the number of CCEs available to the system is N_(CCE) (=└N_(REG)i9┘) and the CCEs are indexed from 0 to N_(CCE)−1.

To simplify the decoding process of a UE, a PDCCH format including n CCEs may start with a CCE having an index equal to a multiple of n. That is, given CCE i, the PDCCH format may start with a CCE satisfying i mod n=0.

The eNB may configure a PDCCH with 1, 2, 4, or 8 CCEs. {1, 2, 4, 8} are called CCE aggregation levels. The number of CCEs used for transmission of a PDCCH is determined according to a channel state by the eNB. For example, one CCE is sufficient for a PDCCH directed to a UE in a good DL channel state (a UE near to the eNB). On the other hand, 8 CCEs may be required for a PDCCH directed to a UE in a poor DL channel state (a UE at a cell edge) in order to ensure sufficient robustness.

[Table 2] below illustrates PDCCH formats. 4 PDCCH formats are supported according to CCE aggregation levels as illustrated in [Table 2].

TABLE 2 Number of Number of Number of PDCCH format CCEs (n) REGs PDCCH bits 0 1 9 72 1 2 18 144 2 4 36 288 3 8 72 576

A different CCE aggregation level is allocated to each UE because the format or Modulation and Coding Scheme (MCS) level of control information delivered in a PDCCH for the UE is different. An MCS level defines a code rate used for data coding and a modulation order. An adaptive MCS level is used for link adaptation. In general, three or four MCS levels may be considered for control channels carrying control information.

Regarding the formats of control information, control information transmitted on a PDCCH is called DCI. The configuration of information in PDCCH payload may be changed depending on the DCI format. The PDCCH payload is information bits. [Table 3] lists DCI according to DCI formats.

TABLE 3 DCI Format Description Format 0 Resource grants for the PUSCH transmissions (uplink) Format 1 Resource assignments for single codeword PDSCH transmissions (transmission modes 1, 2 and 7) Format Compact signaling of resource assignments for single 1A codeword PDSCH (all modes) Format Compact resource assignments for PDSCH using rank-1 1B closed loop precoding (mode 6) Format Very compact resource assignments for PDSCH (e.g. 1C paging/broadcast system information) Format Compact resource assignments for PDSCH using multi-user 1D MIMO (mode 5) Format 2 Resource assignments for PDSCH for closed-loop MIMO operation (mode 4) Format Resource assignments for PDSCH for open-loop MIMO 2A operation (mode 3) Format Power control commands for PUCCH and PUSCH with 3/3A 2-bit/1-bit power adjustment Format 4 Scheduling of PUSCH in one UL cell with multi-antenna port transmission mode

Referring to [Table 3], the DCI formats include Format 0 for PUSCH scheduling, Format 1 for single-codeword PDSCH scheduling, Format 1A for compact single-codeword PDSCH scheduling, Format 1C for very compact DL-SCH scheduling, Format 2 for PDSCH scheduling in a closed-loop spatial multiplexing mode, Format 2A for PDSCH scheduling in an open-loop spatial multiplexing mode, and Format 3/3A for transmission of Transmission Power Control (TPC) commands for uplink channels. DCI Format 1A is available for PDSCH scheduling irrespective of the transmission mode of a UE.

The length of PDCCH payload may vary with DCI formats. In addition, the type and length of PDCCH payload may be changed depending on compact or non-compact scheduling or the transmission mode of a UE.

The transmission mode of a UE may be configured for DL data reception on a PDSCH at the UE. For example, DL data carried on a PDSCH includes scheduled data, a paging message, a random access response, broadcast information on a BCCH, etc. for a UE. The DL data of the PDSCH is related to a DCI format signaled through a PDCCH. The transmission mode may be configured semi-statically for the UE by higher-layer signaling (e.g. Radio Resource Control (RRC) signaling). The transmission mode may be classified as single antenna transmission or multi-antenna transmission.

A transmission mode is configured for a UE semi-statically by higher-layer signaling. For example, multi-antenna transmission scheme may include transmit diversity, open-loop or closed-loop spatial multiplexing, Multi-User Multiple Input Multiple Output (MU-MIMO), or beamforming. Transmit diversity increases transmission reliability by transmitting the same data through multiple Tx antennas. Spatial multiplexing enables high-speed data transmission without increasing a system bandwidth by simultaneously transmitting different data through multiple Tx antennas. Beamforming is a technique of increasing the Signal to Interference plus Noise Ratio (SINR) of a signal by weighting multiple antennas according to channel states.

A DCI format for a UE depends on the transmission mode of the UE. The UE has a reference DCI format monitored according to the transmission mode configure for the UE. The following 10 transmission modes are available to UEs:

(1) Transmission mode 1: Single antenna port (port 0);

(2) Transmission mode 2: Transmit diversity;

(3) Transmission mode 3: Open-loop spatial multiplexing when the number of layer is larger than 1 or Transmit diversity when the rank is 1;

(4) Transmission mode 4: Closed-loop spatial multiplexing;

(5) Transmission mode 5: MU-MIMO;

(6) Transmission mode 6: Closed-loop rank-1 precoding;

(7) Transmission mode 7: Precoding supporting a single layer transmission, which does not based on a codebook (Rel-8);

(8) Transmission mode 8: Precoding supporting up to two layers, which do not based on a codebook (Rel-9);

(9) Transmission mode 9: Precoding supporting up to eight layers, which do not based on a codebook (Rel-10); and

(10) Transmission mode 10: Precoding supporting up to eight layers, which do not based on a codebook, used for CoMP (Rel-11).

1.2.3 PDCCH Transmission

The eNB determines a PDCCH format according to DCI that will be transmitted to the UE and adds a Cyclic Redundancy Check (CRC) to the control information. The CRC is masked by a unique Identifier (ID) (e.g. a Radio Network Temporary Identifier (RNTI)) according to the owner or usage of the PDCCH. If the PDCCH is destined for a specific UE, the CRC may be masked by a unique ID (e.g. a cell-RNTI (C-RNTI)) of the UE. If the PDCCH carries a paging message, the CRC of the PDCCH may be masked by a paging indicator ID (e.g. a Paging-RNTI (P-RNTI)). If the PDCCH carries system information, particularly, a System Information Block (SIB), its CRC may be masked by a system information ID (e.g. a System Information RNTI (SI-RNTI)). To indicate that the PDCCH carries a random access response to a random access preamble transmitted by a UE, its CRC may be masked by a Random Access-RNTI (RA-RNTI).

Then the eNB generates coded data by channel-encoding the CRC-added control information. The channel coding may be performed at a code rate corresponding to an MCS level. The eNB rate-matches the coded data according to a CCE aggregation level allocated to a PDCCH format and generates modulation symbols by modulating the coded data. Herein, a modulation order corresponding to the MCS level may be used for the modulation. The CCE aggregation level for the modulation symbols of a PDCCH may be one of 1, 2, 4, and 8. Subsequently, the eNB maps the modulation symbols to physical REs (i.e. CCE to RE mapping).

1.2.4 Blind Decoding (BD)

A plurality of PDCCHs may be transmitted in a subframe. That is, the control region of a subframe includes a plurality of CCEs, CCE 0 to CCE N_(CCE,k)−1. N_(CCE,k) is the total number of CCEs in the control region of a k^(th) subframe. A UE monitors a plurality of PDCCHs in every subframe. This means that the UE attempts to decode each PDCCH according to a monitored PDCCH format.

The eNB does not provide the UE with information about the position of a PDCCH directed to the UE in an allocated control region of a subframe. Without knowledge of the position, CCE aggregation level, or DCI format of its PDCCH, the UE searches for its PDCCH by monitoring a set of PDCCH candidates in the subframe in order to receive a control channel from the eNB. This is called blind decoding. Blind decoding is the process of demasking a CRC part with a UE ID, checking a CRC error, and determining whether a corresponding PDCCH is a control channel directed to a UE by the UE.

The UE monitors a PDCCH in every subframe to receive data transmitted to the UE in an active mode. In a Discontinuous Reception (DRX) mode, the UE wakes up in a monitoring interval of every DRX cycle and monitors a PDCCH in a subframe corresponding to the monitoring interval. The PDCCH-monitored subframe is called a non-DRX subframe.

To receive its PDCCH, the UE should blind-decode all CCEs of the control region of the non-DRX subframe. Without knowledge of a transmitted PDCCH format, the UE should decode all PDCCHs with all possible CCE aggregation levels until the UE succeeds in blind-decoding a PDCCH in every non-DRX subframe. Since the UE does not know the number of CCEs used for its PDCCH, the UE should attempt detection with all possible CCE aggregation levels until the UE succeeds in blind decoding of a PDCCH.

In the LTE system, the concept of Search Space (SS) is defined for blind decoding of a UE. An SS is a set of PDCCH candidates that a UE will monitor. The SS may have a different size for each PDCCH format. There are two types of SSs, Common Search Space (CSS) and UE-specific/Dedicated Search Space (USS).

While all UEs may know the size of a CSS, a USS may be configured for each individual UE. Accordingly, a UE should monitor both a CSS and a USS to decode a PDCCH. As a consequence, the UE performs up to 44 blind decodings in one subframe, except for blind decodings based on different CRC values (e.g., C-RNTI, P-RNTI, SI-RNTI, and RA-RNTI).

In view of the constraints of an SS, the eNB may not secure CCE resources to transmit PDCCHs to all intended UEs in a given subframe. This situation occurs because the remaining resources except for allocated CCEs may not be included in an SS for a specific UE. To minimize this obstacle that may continue in the next subframe, a UE-specific hopping sequence may apply to the starting point of a USS.

[Table 4] illustrates the sizes of CSSs and USSs.

TABLE 4 Number of Number of candidates candidates Number of CCEs in common in dedicated PDCCH format (n) search space search space 0 1 — 6 1 2 — 6 2 4 4 2 3 8 2 2

To mitigate the load of the UE caused by the number of blind decoding attempts, the UE does not search for all defined DCI formats simultaneously. Specifically, the UE always searches for DCI Format 0 and DCI Format 1A in a USS. Although DCI Format 0 and DCI Format 1A are of the same size, the UE may distinguish the DCI formats by a flag for format0/format 1a differentiation included in a PDCCH. Other DCI formats than DCI Format 0 and DCI Format 1A, such as DCI Format 1, DCI Format 1B, and DCI Format 2 may be required for the UE.

The UE may search for DCI Format 1A and DCI Format 1C in a CSS. The UE may also be configured to search for DCI Format 3 or 3A in the CSS. Although DCI Format 3 and DCI Format 3A have the same size as DCI Format 0 and DCI Format 1A, the UE may distinguish the DCI formats by a CRC scrambled with an ID other than a UE-specific ID.

An SS S_(k) ^((L)) is a PDCCH candidate set with a CCE aggregation level L∈{1, 2, 4, 8}. The CCEs of PDCCH candidate set m in the SS may be determined by the following equation. L·{(Y _(k) +m)mod└N _(CCE,k) /L┘}+i  [Equation 1]

where M^((L)) is the number of PDCCH candidates with CCE aggregation level L to be monitored in the SS, m=0, . . . , M^((L))−1, i is the index of a CCE in each PDCCH candidate, and i=0, . . . , L−1, k=└n_(s)/2┘ where n_(s) is the index of a slot in a radio frame.

As described before, the UE monitors both the USS and the CSS to decode a PDCCH. The CSS supports PDCCHs with CCE aggregation levels {4, 8} and the USS supports PDCCHs with CCE aggregation levels {1, 2, 4, 8}. [Table 5] illustrates PDCCH candidates monitored by a UE.

TABLE 5 Search space S_(k) ^((L)) Number of PDCCH Type Aggregation level L Size [in CCEs] candidates M^((L)) UE- 1 6 6 specific 2 12 6 4 8 2 8 16 2 Common 4 16 4 8 16 2

Referring to [Equation 1], for two aggregation levels, L=4 and L=8, Y_(k) is set to 0 in the CSS, whereas Y₄ is defined by [Equation 2] for aggregation level L in the USS. Y _(k)=(A·Y _(k−1))mod D  [Equation 2]

where Y⁻¹=n_(RNTI)≠0, n_(RNTI) indicating an RNTI value. A=39827 and D=65537.

1.3. PUCCH (Physical Uplink Control Channel)

PUCCH may include the following formats to transmit control information.

(1) Format 1: On-Off keying (OOK) modulation, used for SR (Scheduling Request)

(2) Format 1a & 1b: Used for ACK/NACK transmission

-   -   1) Format 1a: BPSK ACK/NACK for 1 codeword     -   2) Format 1b: QPSK ACK/NACK for 2 codewords

(3) Format 2: QPSK modulation, used for CQI transmission

(4) Format 2a & Format 2b: Used for simultaneous transmission of CQI and ACK/NACK

(5) Format 3: Used for multiple ACK/NACK transmission in a carrier aggregation environment

Table 6 shows a modulation scheme according to PUCCH format and the number of bits per subframe. Table 7 shows the number of reference signals (RS) per slot according to PUCCH format. Table 8 shows SC-FDMA symbol location of RS (reference signal) according to PUCCH format. In Table 6, PUCCH format 2a and PUCCH format 2b correspond to a case of normal cyclic prefix (CP).

TABLE 6 PUCCH format Modulation scheme No. of bits per subframe, Mbit 1  N/A N/A 1a BPSK 1 1b QPSK 2 2  QPSK 20 2a QPSK + BPSK 21 2b QPSK + BPSK 22 3  QPSK 48

TABLE 7 PUCCH format Normal CP Extended CP 1, 1a, 1b 3 2 2, 3 2 1 2a, 2b 2 N/A

TABLE 8 SC-FDMA symbol location of RS PUCCH format Normal CP Extended CP 1, 1a, 1b 2, 3, 4 2, 3 2, 3 1, 5 3 2a, 2b 1, 5 N/A

FIG. 6 shows PUCCH formats 1a and 1b in case of a normal cyclic prefix. And, FIG. 7 shows PUCCH formats 1a and 1b in case of an extended cyclic prefix.

According to the PUCCH formats 1a and 1b, control information of the same content is repeated in a subframe by slot unit. In each user equipment, ACK/NACK signal is transmitted on a different resource constructed with a different cyclic shift (CS) (frequency domain code) and an orthogonal cover (OC) or orthogonal cover code (OCC) (time domain spreading code) of CG-CAZAC (computer-generated constant amplitude zero auto correlation) sequence. For instance, the OC includes Walsh/DFT orthogonal code. If the number of CS and the number of OC are 6 and 3, respectively, total 18 user equipments may be multiplexed within the same PRB (physical resource block) with reference to a single antenna. Orthogonal sequences w0, w1, w2 and w3 may be applicable to a random time domain (after FFT modulation) or a random frequency domain (before FFT modulation).

For persistent scheduling with SR, ACK/NACK resource constructed with CS, OC and PRB (physical resource block) may be allocated to a user equipment through RRC (radio resource control. For non-persistent scheduling with dynamic ACK/NACK, the ACK/NACK resource may be implicitly allocated to a user equipment using a smallest CCE index of PDCCH corresponding to PDSCH.

Length-4 orthogonal sequence (OC) and length-3 orthogonal sequence for PUCCH format 1/1a/1b are shown in Table 9 and Table 10, respectively.

TABLE 9 Orthogonal sequences Sequence index n_(oc)(n_(s)) [w(0) . . . w(N_(SF) ^(PUCCH) − 1)] 0 [+1 +1 +1 +1] 1 [+1 −1 +1 −1] 2 [+1 −1 −1 +1]

TABLE 10 Orthogonal sequences Sequence index n_(oc)(n_(s)) [w(0) . . . w(N_(SF) ^(PUCCH) − 1)] 0 [1 1 1] 1 [1 e^(j2π/3) e^(j4π/3)] 2 [1 e^(j4π/3) e^(j2π/3)]

Orthogonal sequence (OC) [w(0) . . . w(N_(RS) ^(PUCCH)−1)] for a reference signal in PUCCH format 1/1a/1b is shown in Table 11.

TABLE 11 Sequence index n _(oc)(n_(s)) Normal cyclic prefix Extended cyclic prefix 0 [1 1 1] [1 1] 1 [1 e^(j2π/3) e^(j4π/3)] [1 −1] 2 [1 e^(j4π/3) e^(j2π/3)] N/A

FIG. 8 shows PUCCH format 2/2a/2b in case of a normal cyclic prefix. And, FIG. 9 shows PUCCH format 2/2a/2b in case of an extended cyclic prefix.

Referring to FIG. 8 and FIG. 9, in case of a normal CP, a subframe is constructed with 10 QPSK data symbols as well as RS symbol. Each QPSK symbol is spread in a frequency domain by CS and is then mapped to a corresponding SC-FDMA symbol. SC-FDMA symbol level CS hopping may be applied to randomize inter-cell interference. The RS may be multiplexed by CDM using a cyclic shift. For instance, assuming that the number of available CSs is 12, 12 user equipments may be multiplexed in the same PRB. For instance, assuming that the number of available CSs is 6, 6 user equipments may be multiplexed in the same PRB. In brief, a plurality of user equipments in PUCCH format 1/1a/1b and PUCCH format 2/2a/2b may be multiplexed by ‘CS+OC+PRB’ and ‘CS+PRB’, respectively.

FIG. 10 is a diagram of ACK/NACK channelization for PUCCH formats 1a and 1b. In particular, FIG. 10 corresponds to a case of ‘Δ_(shift) ^(PUCCH)=2’

FIG. 11 is a diagram of channelization for a hybrid structure of PUCCH format 1/1a/1b and PUCCH format 2/2a/2b.

Cyclic shift (CS) hopping and orthogonal cover (OC) remapping may be applicable in a following manner.

(1) Symbol-based cell-specific CS hopping for randomization of inter-cell interference

(2) Slot level CS/OC remapping

-   -   1) For inter-cell interference randomization     -   2) Slot based access for mapping between ACK/NACK channel and         resource (k)

Meanwhile, resource n_(r) for PUCCH format 1/1a/1b may include the following combinations.

(1) CS (=equal to DFT orthogonal code at symbol level) (n_(cs))

(2) OC (orthogonal cover at slot level) (n_(oc))

(3) Frequency RB (Resource Block) (n_(rb))

If indexes indicating CS, OC and RB are set to n_(cs), n_(oc), n_(rb), respectively, a representative index n_(r) may include n_(cs), n_(oc), and n_(rb). In this case, the n_(r) may meet the condition of ‘n_(r)=(n_(cs), n_(oc), n_(rb))’.

The combination of CQI, PMI, RI, CQI and ACK/NACK may be delivered through the PUCCH format 2/2a/2b. And, Reed Muller (RM) channel coding may be applicable.

For instance, channel coding for UL (uplink) CQI in LTE system may be described as follows. First of all, bitstreams a₀, a₁, a₂, a₃, . . . , a_(A−1) may be coded using (20, A) RM code. In this case, a⁰ and a_(A−1) indicates MSB (Most Significant Bit) and LSB (Least Significant Bit), respectively. In case of an extended cyclic prefix, maximum information bits include 11 bits except a case that QI and ACK/NACK are simultaneously transmitted. After coding has been performed with 20 bits using the RM code, QPSK modulation may be applied. Before the BPSK modulation, coded bits may be scrambled.

Table 12 shows a basic sequence for (20, A) code.

TABLE 12 i M_(i, 0) M_(i, 1) M_(i, 2) M_(i, 3) M_(i, 4) M_(i, 5) M_(i, 6) M_(i, 7) M_(i, 8) M_(i, 9) M_(i, 10) M_(i, 11) M_(i, 12) 0 1 1 0 0 0 0 0 0 0 0 1 1 0 1 1 1 1 0 0 0 0 0 0 1 1 1 0 2 1 0 0 1 0 0 1 0 1 1 1 1 1 3 1 0 1 1 0 0 0 0 1 0 1 1 1 4 1 1 1 1 0 0 0 1 0 0 1 1 1 5 1 1 0 0 1 0 1 1 1 0 1 1 1 6 1 0 1 0 1 0 1 0 1 1 1 1 1 7 1 0 0 1 1 0 0 1 1 0 1 1 1 8 1 1 0 1 1 0 0 1 0 1 1 1 1 9 1 0 1 1 1 0 1 0 0 1 1 1 1 10 1 0 1 0 0 1 1 1 0 1 1 1 1 11 1 1 1 0 0 1 1 0 1 0 1 1 1 12 1 0 0 1 0 1 0 1 1 1 1 1 1 13 1 1 0 1 0 1 0 1 0 1 1 1 1 14 1 0 0 0 1 1 0 1 0 0 1 0 1 15 1 1 0 0 1 1 1 1 0 1 1 0 1 16 1 1 1 0 1 1 1 0 0 1 0 1 1 17 1 0 0 1 1 1 0 0 1 0 0 1 1 18 1 1 0 1 1 1 1 1 0 0 0 0 0 19 1 0 0 0 0 1 1 0 0 0 0 0 0

Channel coding bits b₀, b₁, b₂, b₃, . . . , b_(B−1) may be generated by Formula 1.

$\begin{matrix} {b_{i} = {\sum\limits_{n = 0}^{A - 1}\;{\left( {a_{n} \cdot M_{i,n}} \right){mod}\mspace{14mu} 2}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack \end{matrix}$

In Formula 3, ‘i=0, 1, 2, . . . , B−1’ is met.

In case of wideband repots, a bandwidth of UCI (uplink control information) field for CQI/PMI can be represented as Tables 8 to 10 in the following.

Table 13 shows UCI (Uplink Control Information) field for broadband report (single antenna port, transmit diversity) or open loop spatial multiplexing PDSCH CQI feedback.

TABLE 13 Field Bandwidth Broadband CQI 4

Table 14 shows UL control information (UCI) field for CQI and PMI feedback in case of wideband reports (closed loop spatial multiplexing PDSCH transmission).

TABLE 14 Bandwidth 2 antenna ports 4 antenna ports Field rank = 1 rank = 2 rank = 1 Rank > 1 Wideband CQI 4 4 4 4 Spatial differential CQI 0 3 0 3 Precoding Matrix 2 1 4 4 Indication

Table 15 shows UL control information (UCI) field for RI feedback in case of wideband reports.

TABLE 15 Bit widths 4 antenna ports Field 2 antenna ports Max. 2 layers Max. 4 layers Rank Indication 1 1 2

FIG. 12 is a diagram for PRB allocation. Referring to FIG. 20, PRB may be usable for PUCCH transmission in a slot n_(s).

2. Carrier Aggregation (CA) Environment

2.1 CA Overview

A 3GPP LTE system (conforming to Rel-8 or Rel-9) (hereinafter, referred to as an LTE system) uses Multi-Carrier Modulation (MCM) in which a single Component Carrier (CC) is divided into a plurality of bands. In contrast, a 3GPP LTE-A system (hereinafter, referred to an LTE-A system) may use CA by aggregating one or more CCs to support a broader system bandwidth than the LTE system. The term CA is interchangeably used with carrier combining, multi-CC environment, or multi-carrier environment.

In the present disclosure, multi-carrier means CA (or carrier combining). Herein, CA covers aggregation of contiguous carriers and aggregation of non-contiguous carriers. The number of aggregated CCs may be different for a DL and a UL. If the number of DL CCs is equal to the number of UL CCs, this is called symmetric aggregation. If the number of DL CCs is different from the number of UL CCs, this is called asymmetric aggregation. The term CA is interchangeable with carrier combining, bandwidth aggregation, spectrum aggregation, etc.

The LTE-A system aims to support a bandwidth of up to 100 MHz by aggregating two or more CCs, that is, by CA. To guarantee backward compatibility with a legacy IMT system, each of one or more carriers, which has a smaller bandwidth than a target bandwidth, may be limited to a bandwidth used in the legacy system.

For example, the legacy 3GPP LTE system supports bandwidths {1.4, 3, 5, 10, 15, and 20 MHz} and the 3GPP LTE-A system may support a broader bandwidth than 20 MHz using these LTE bandwidths. A CA system of the present disclosure may support CA by defining a new bandwidth irrespective of the bandwidths used in the legacy system.

There are two types of CA, intra-band CA and inter-band CA. Intra-band CA means that a plurality of DL CCs and/or UL CCs are successive or adjacent in frequency. In other words, the carrier frequencies of the DL CCs and/or UL CCs are positioned in the same band. On the other hand, an environment where CCs are far away from each other in frequency may be called inter-band CA. In other words, the carrier frequencies of a plurality of DL CCs and/or UL CCs are positioned in different bands. In this case, a UE may use a plurality of Radio Frequency (RF) ends to conduct communication in a CA environment.

The LTE-A system adopts the concept of cell to manage radio resources. The above-described CA environment may be referred to as a multi-cell environment. A cell is defined as a pair of DL and UL CCs, although the UL resources are not mandatory. Accordingly, a cell may be configured with DL resources alone or DL and UL resources.

For example, if one serving cell is configured for a specific UE, the UE may have one DL CC and one UL CC. If two or more serving cells are configured for the UE, the UE may have as many DL CCs as the number of the serving cells and as many UL CCs as or fewer UL CCs than the number of the serving cells, or vice versa. That is, if a plurality of serving cells are configured for the UE, a CA environment using more UL CCs than DL CCs may also be supported.

CA may be regarded as aggregation of two or more cells having different carrier frequencies (center frequencies). Herein, the term ‘cell’ should be distinguished from ‘cell’ as a geographical area covered by an eNB. Hereinafter, intra-band CA is referred to as intra-band multi-cell and inter-band CA is referred to as inter-band multi-cell.

In the LTE-A system, a Primacy Cell (PCell) and a Secondary Cell (SCell) are defined. A PCell and an SCell may be used as serving cells. For a UE in RRC_CONNECTED state, if CA is not configured for the UE or the UE does not support CA, a single serving cell including only a PCell exists for the UE. On the contrary, if the UE is in RRC_CONNECTED state and CA is configured for the UE, one or more serving cells may exist for the UE, including a PCell and one or more SCells.

Serving cells (PCell and SCell) may be configured by an RRC parameter. A physical-layer ID of a cell, PhysCellId is an integer value ranging from 0 to 503. A short ID of an SCell, SCellIndex is an integer value ranging from 1 to 7. A short ID of a serving cell (PCell or SCell), ServeCellIndex is an integer value ranging from 1 to 7. If ServeCellIndex is 0, this indicates a PCell and the values of ServeCellIndex for SCells are pre-assigned. That is, the smallest cell ID (or cell index) of ServeCellIndex indicates a PCell.

A PCell refers to a cell operating in a primary frequency (or a primary CC). A UE may use a PCell for initial connection establishment or connection reestablishment. The PCell may be a cell indicated during handover. In addition, the PCell is a cell responsible for control-related communication among serving cells configured in a CA environment. That is, PUCCH allocation and transmission for the UE may take place only in the PCell. In addition, the UE may use only the PCell in acquiring system information or changing a monitoring procedure. An Evolved Universal Terrestrial Radio Access Network (E-UTRAN) may change only a PCell for a handover procedure by a higher-layer RRCConnectionReconfiguraiton message including mobilityControlInfo to a UE supporting CA.

An SCell may refer to a cell operating in a secondary frequency (or a secondary CC). Although only one PCell is allocated to a specific UE, one or more SCells may be allocated to the UE. An SCell may be configured after RRC connection establishment and may be used to provide additional radio resources. There is no PUCCH in cells other than a PCell, that is, in SCells among serving cells configured in the CA environment.

When the E-UTRAN adds an SCell to a UE supporting CA, the E-UTRAN may transmit all system information related to operations of related cells in RRC_CONNECTED state to the UE by dedicated signaling. Changing system information may be controlled by releasing and adding a related SCell. Herein, a higher-layer RRCConnectionReconfiguration message may be used. The E-UTRAN may transmit a dedicated signal having a different parameter for each cell rather than it broadcasts in a related SCell.

After an initial security activation procedure starts, the E-UTRAN may configure a network including one or more SCells by adding the SCells to a PCell initially configured during a connection establishment procedure. In the CA environment, each of a PCell and an SCell may operate as a CC. Hereinbelow, a Primary CC (PCC) and a PCell may be used in the same meaning and a Secondary CC (SCC) and an SCell may be used in the same meaning in embodiments of the present disclosure.

FIG. 13 illustrates an example of CCs and CA in the LTE-A system, which are used in embodiments of the present disclosure.

FIG. 13(a) illustrates a single carrier structure in the LTE system. There are a DL CC and a UL CC and one CC may have a frequency range of 20 MHz.

FIG. 13(b) illustrates a CA structure in the LTE-A system. In the illustrated case of FIG. 13(b), three CCs each having 20 MHz are aggregated. While three DL CCs and three UL CCs are configured, the numbers of DL CCs and UL CCs are not limited. In CA, a UE may monitor three CCs simultaneously, receive a DL signal/DL data in the three CCs, and transmit a UL signal/UL data in the three CCs.

If a specific cell manages N DL CCs, the network may allocate M (M≤N) DL CCs to a UE. The UE may monitor only the M DL CCs and receive a DL signal in the M DL CCs. The network may prioritize L (L≤M≤N) DL CCs and allocate a main DL CC to the UE. In this case, the UE should monitor the L DL CCs. The same thing may apply to UL transmission.

The linkage between the carrier frequencies of DL resources (or DL CCs) and the carrier frequencies of UL resources (or UL CCs) may be indicated by a higher-layer message such as an RRC message or by system information. For example, a set of DL resources and UL resources may be configured based on linkage indicated by System Information Block Type 2 (SIB2). Specifically, DL-UL linkage may refer to a mapping relationship between a DL CC carrying a PDCCH with a UL grant and a UL CC using the UL grant, or a mapping relationship between a DL CC (or a UL CC) carrying HARQ data and a UL CC (or a DL CC) carrying an HARQ ACK/NACK signal.

2.2 Cross Carrier Scheduling

Two scheduling schemes, self-scheduling and cross carrier scheduling are defined for a CA system, from the perspective of carriers or serving cells. Cross carrier scheduling may be called cross CC scheduling or cross cell scheduling.

In self-scheduling, a PDCCH (carrying a DL grant) and a PDSCH are transmitted in the same DL CC or a PUSCH is transmitted in a UL CC linked to a DL CC in which a PDCCH (carrying a UL grant) is received.

In cross carrier scheduling, a PDCCH (carrying a DL grant) and a PDSCH are transmitted in different DL CCs or a PUSCH is transmitted in a UL CC other than a UL CC linked to a DL CC in which a PDCCH (carrying a UL grant) is received.

Cross carrier scheduling may be activated or deactivated UE-specifically and indicated to each UE semi-statically by higher-layer signaling (e.g. RRC signaling).

If cross carrier scheduling is activated, a Carrier Indicator Field (CIF) is required in a PDCCH to indicate a DL/UL CC in which a PDSCH/PUSCH indicated by the PDCCH is to be transmitted. For example, the PDCCH may allocate PDSCH resources or PUSCH resources to one of a plurality of CCs by the CIF. That is, when a PDCCH of a DL CC allocates PDSCH or PUSCH resources to one of aggregated DL/UL CCs, a CIF is set in the PDCCH. In this case, the DCI formats of LTE Release-8 may be extended according to the CIF. The CIF may be fixed to three bits and the position of the CIF may be fixed irrespective of a DCI format size. In addition, the LTE Release-8 PDCCH structure (the same coding and resource mapping based on the same CCEs) may be reused.

On the other hand, if a PDCCH transmitted in a DL CC allocates PDSCH resources of the same DL CC or allocates PUSCH resources in a single UL CC linked to the DL CC, a CIF is not set in the PDCCH. In this case, the LTE Release-8 PDCCH structure (the same coding and resource mapping based on the same CCEs) may be used.

If cross carrier scheduling is available, a UE needs to monitor a plurality of PDCCHs for DCI in the control region of a monitoring CC according to the transmission mode and/or bandwidth of each CC. Accordingly, an appropriate SS configuration and PDCCH monitoring are needed for the purpose.

In the CA system, a UE DL CC set is a set of DL CCs scheduled for a UE to receive a PDSCH, and a UE UL CC set is a set of UL CCs scheduled for a UE to transmit a PUSCH. A PDCCH monitoring set is a set of one or more DL CCs in which a PDCCH is monitored. The PDCCH monitoring set may be identical to the UE DL CC set or may be a subset of the UE DL CC set. The PDCCH monitoring set may include at least one of the DL CCs of the UE DL CC set. Or the PDCCH monitoring set may be defined irrespective of the UE DL CC set. DL CCs included in the PDCCH monitoring set may be configured to always enable self-scheduling for UL CCs linked to the DL CCs. The UE DL CC set, the UE UL CC set, and the PDCCH monitoring set may be configured UE-specifically, UE group-specifically, or cell-specifically.

If cross carrier scheduling is deactivated, this implies that the PDCCH monitoring set is always identical to the UE DL CC set. In this case, there is no need for signaling the PDCCH monitoring set. However, if cross carrier scheduling is activated, the PDCCH monitoring set may be defined within the UE DL CC set. That is, the eNB transmits a PDCCH only in the PDCCH monitoring set to schedule a PDSCH or PUSCH for the UE.

FIG. 14 illustrates a cross carrier-scheduled subframe structure in the LTE-A system, which is used in embodiments of the present disclosure.

Referring to FIG. 14, three DL CCs are aggregated for a DL subframe for LTE-A UEs. DL CC ‘A’ is configured as a PDCCH monitoring DL CC. If a CIF is not used, each DL CC may deliver a PDCCH that schedules a PDSCH in the same DL CC without a CIF. On the other hand, if the CIF is used by higher-layer signaling, only DL CC ‘A’ may carry a PDCCH that schedules a PDSCH in the same DL CC ‘A’ or another CC. Herein, no PDCCH is transmitted in DL CC ‘B’ and DL CC ‘C’ that are not configured as PDCCH monitoring DL CCs.

FIG. 15 is conceptual diagram illustrating a construction of serving cells according to cross-carrier scheduling.

Referring to FIG. 15, an eNB (or BS) and/or UEs for use in a radio access system supporting carrier aggregation (CA) may include one or more serving cells. In FIG. 8, the eNB can support a total of four serving cells (cells A, B, C and D). It is assumed that UE A may include Cells (A, B, C), UE B may include Cells (B, C, D), and UE C may include Cell B. In this case, at least one of cells of each UE may be composed of P Cell. In this case, P Cell is always activated, and S Cell may be activated or deactivated by the eNB and/or UE.

The cells shown in FIG. 15 may be configured per UE. The above-mentioned cells selected from among cells of the eNB, cell addition may be applied to carrier aggregation (CA) on the basis of a measurement report message received from the UE. The configured cell may reserve resources for ACK/NACK message transmission in association with PDSCH signal transmission. The activated cell is configured to actually transmit a PDSCH signal and/or a PUSCH signal from among the configured cells, and is configured to transmit CSI reporting and Sounding Reference Signal (SRS) transmission. The deactivated cell is configured not to transmit/receive PDSCH/PUSCH signals by an eNB command or a timer operation, and CRS reporting and SRS transmission are interrupted.

2.3 CA PUCCH (Carrier Aggregation Physical Uplink Control Channel)

In a wireless communication system supportive of carrier aggregation, PUCCH format for feeding back UCI (e.g., multi-ACK/NACK bit) can be defined. For clarity of the following description, such PUCCH format shall be named CA PUCCH format.

FIG. 16 is a diagram for one example of a signal processing process of CA PUCCH.

Referring to FIG. 16, a channel coding block generates coding bits (e.g., encoded bits, coded bits, etc.) (or codeword) b_0, b_1, . . . and b_N−1 by channel-coding information bits a_0, a_1, . . . and a_M−1 (e.g., multiple ACK/NACK bits). In this case, the M indicates a size of information bits and the N indicates a size of the coding bits. The information bits may include multiple ACK/NACK for UL control information (UCI), e.g., a plurality of data (or PDSCH) received via a plurality of DL CCS. In this case, the information bits a_0, a_1, . . . a_M−1 may be joint-coded irrespective of type/number/size of the UCI configuring the information bits. For instance, in case that information bits include multiple ACK/NACK for a plurality of DL CCs, channel coding may not be performed per DL CC or individual ACK/NACK bit but may be performed on all bit information, from which a single codeword may be generated. And, channel coding is non-limited by this. Moreover, the channel coding may include one of simplex repetition, simplex coding, RM (Reed Muller) coding, punctured RM coding, TBCC (tail-biting convolutional coding), LDPC (low-density parity-check), turbo coding and the like. Besides, coding bits may be rate-matched in consideration of a modulation order and a resource size (not shown in the drawing). A rate matching function may be included as a part of the channel coding block or may be performed via a separate function block.

A modulator generates modulated symbols c_0, c_1 . . . c_L−1 by modulating coding bits b_0, b_1 . . . b_N−1. In this case, the L indicates a size of modulated symbol. This modulation scheme may be performed in a manner of modifying a size and phase of a transmission signal. For instance, the modulation scheme may include one of n-PSK (Phase Shift Keying), n-QAM (Quadrature Amplitude Modulation) and the like, where n is an integer equal to or greater than 2. In particular, the modulation scheme may include one of BPSK (Binary PSK), QPSK (Quadrature PSK), 8-PSK, QAM, 16-QAM, 64-QAM and the like.

A divider divides the modulated symbols c_0, c_1 . . . c_L−1 to slots, respectively. A sequence/pattern/scheme for dividing the modulated symbols to the slots may be specially non-limited. For instance, the divider may be able to divide the modulated symbols to the corresponding slots in order from a head to tail (Localized scheme). In doing so, as shown in the drawing, the modulated symbols c_0, c_1 . . . c_L/2−1 may be divided to the slot 0 and the modulated symbols c_L/2, c_L/2+1 . . . c_L−1 may be divided to the slot 1. Moreover, the modulated symbols may be divided to the corresponding slots, respectively, by interleaving or permutation. For instance, the even-numbered modulated symbol may be divided to the slot 0, while the odd-numbered modulated symbol may be divided to the slot 1. The modulation scheme and the dividing scheme may be switched to each other in order.

A DFT precoder may perform DFT precoding (e.g., 12-point DFT) on the modulated symbols divided to the corresponding slots to generate a single carrier waveform. Referring to the drawing, the modulated symbols c_0, c_1 . . . c_L/2−1 divided to the corresponding slot 0 may be DFT-precoded into DFT symbols d_0, d_1 . . . d_L/2−1, and the modulated symbols c_L/2, c_L/2+1 . . . c_L−1 divided to the slot 1 may be DFT-precoded into DFT symbols d_L/2, d_L/2+1 . . . d_L−1. Moreover, the DFT precoding may be replaced by another linear operation (e.g., Walsh precoding) corresponding thereto.

A spreading block may spread the DFT-performed signal at SC-FDMA symbols level (e.g., time domain). The time-domain spreading at the SC-FDMA level may be performed using a spreading code (sequence). The spreading code may include pseudo orthogonal code and orthogonal code. The pseudo orthogonal code may include PN (pseudo noise) code, by which the pseudo orthogonal code may be non-limited. The orthogonal code may include Walsh code and DFT code, by which the orthogonal code may be non-limited. The orthogonal code (OC) may be interchangeably used with one of an orthogonal sequence, an orthogonal cover (OC) and an orthogonal cover code (OCC). In this specification, for example, the orthogonal code may be mainly described as a representative example of the spreading code for clarity and convenience of the following description. Optionally, the orthogonal code may be substituted with the pseudo orthogonal code. A maximum value of a spreading code size (or a spreading factor: SF) may be limited by the number of SC-FDMA symbols used for control information transmission. For example, in case that 5 SC-FDMA symbols are used in one slot for control information transmission, orthogonal codes (or pseudo orthogonal codes) w0, w1, w2, w3 and w4 of length 5 may be used per slot. The SF may mean a spreading degree of the control information and may be associated with a multiplexing order or an antenna multiplexing order of a user equipment. The SF may be variable like 1, 2, 3, 4, 5 . . . depending on a requirement of a system. The SF may be defined in advance between a base station and a user equipment. And, the SF may be notified to a user equipment via DCI or RRC signaling.

The signal generated through the above-described process may be mapped to subcarrier within the PRB and may be then transformed into a time-domain signal through IFFT. CP may be attached to the time-domain signal. The generated SC-FDMA symbol may be then transmitted through an RF stage.

2. 4 CSI (Channel State Information) Feedback through PUCCH

First of all, in the 3GPP LTE system, when a DL reception entity (e.g., a user equipment) is connected to a DL transmission entity (e.g., a base station), the DL reception entity performs measurement on a reference signal received power (RSRP) of a reference signal transmitted in DL, a quality of a reference signal (RSRQ: reference signal received quality) and the like at a random time and is then able to make a periodic or even-triggered report of a corresponding measurement result to the base station.

Each user equipment reports a DL channel information in accordance with a DL channel status via uplink. A base station is then able to determine time/frequency resources, MCS (modulation and coding scheme) and the like appropriate for a data transmission to each user equipment using the DL channel information received from the each user equipment.

Such channel state information (CSI) may include CQI (Channel Quality Indication), PMI (Precoding Matrix Indicator), PTI (Precoder Type Indication) and/or RI (Rank Indication). In particular, the CSI may be transmitted entirely or partially depending on a transmission mode of each user equipment. CQI is determined based on a received signal quality of a user equipment, which may be generally determined on the basis of a measurement of a DL reference signal. In doing so, a CQI value actually delivered to a base station may correspond to an MCS capable of providing maximum performance by maintaining a block error rate (BLER) under 10% in the received signal quality measured by a user equipment.

This channel information reporting may be classified into a periodic report transmitted periodically and an aperiodic report transmitted in response to a request made by a base station.

In case of the aperiodic report, it is set for each user equipment by a 1-bit request bit (CQI request bit) contained in UL scheduling information downloaded to a user equipment by a base station. Having received this information, each user equipment is then able to deliver channel information to the base station via a physical uplink shared channel (PUSCH) in consideration of its transmission mode. And, it may set RI and CQI/PMI not to be transmitted on the same PUSCH.

In case of the periodic report, a period for transmitting channel information via an upper layer signal, an offset in the corresponding period and the like are signaled to each user equipment by subframe unit and channel information in consideration of a transmission mode of each user equipment may be delivered to a base station via a physical uplink control channel (PUCCH) in accordance with a determined period. In case that data transmitted in uplink simultaneously exists in a subframe in which channel information is transmitted by a determined period, the corresponding channel information may be transmitted together with the data not on the physical uplink control channel (PUCCH) but on a physical uplink shared channel (PUSCH). In case of the periodic report via PUCCH, bits (e.g., 11 bits) limited further than those of the PUSCH may be used. RI and CQI/PMI may be transmitted on the same PUSCH.

In case that contention occurs between the periodic report and the aperiodic report in the same subframe, only the aperiodic report can be performed.

In calculating Wideband CQI/PMI, a most recently transmitted RI may be usable. RI in a PUCCH CSI report mode is independent from RI in a PUSCH CSI report mode. The RI in the PUSCH CSI report mode is valid for CQI/PMI in the corresponding PUSCH CSI report mode only.

Table 16 is provided to describe CSI feedback type transmitted on PUCCH and PUCCH CSI report mode.

TABLE 16 PMI Feedback Type No PMI (OL, TD, single-antenna) Single PMI (CL) CQI Wideband Mode 1-0 Mode 1-1 Feedback RI (only for Open-Loop SM) RI Type One Wideband CQI (4 bit) Wideband CQI (4 bit) when RI > 1, CQI of first codeword Wideband spatial CQI (3 bit) for RI > 1 Wideband PMI (4 bit) UE Mode 2-0 Mode 2-1 Selected RI (only for Open-Loop SM) RI Wideband CQI (4 bit) Wideband CQI (4 bit) Best-1 CQI (4 bit) in each BP Wideband spatial CQI (3 bit) for RI > 1 Best-1 indicator(L-bit label) Wideband PMI (4 bit) when RI > 1, CQI of first eodeword Best-1 CQI (4 bit) 1 in each BP Best-1 spatial CQI (3 bit) for RI > 1 Best-1 indicator (L-bit label)

Referring to Table 16, in the periodic report of channel information, there are 4 kinds of reporting modes (mode 1-0, mode 1-2, mode 2-0 and mode 2-1) in accordance with CQI and PMI feedback types.

CQI can be classified into WB (wideband) CQI and SB (subband) CQI in accordance with CQI feedback type and PMI can be classified into No PMI or Single PMI in accordance with a presence or non-presence of PMI transmission. In Table 11, No PMI corresponds to a case of open-loop (OL), transmit diversity (TD) and single-antenna, while Single PMI corresponds to a case of closed-loop (CL).

The mode 1-0 corresponds to a case that WB CQI is transmitted in the absence of PMI transmission. In this case, RI is transmitted only in case of open-loop (OL) spatial multiplexing (SM) and one WB CQI represented as 4 bits can be transmitted. If RI is greater than 1, CQI for a 1^(st) codeword can be transmitted.

The mode 1-1 corresponds to a case that a single PMI and WB CQI are transmitted. In this case, 4-bit WB CQI and 4-bit WB PMI can be transmitted together with RI transmission. Additionally, if RI is greater than 1, 3-bit WB (wideband) spatial differential CQI can be transmitted. In 2-codeword transmission, the WB spatial differential CQI may indicate a difference value between a WB CQI index for codeword 1 and a WB CQI index for codeword 2. The difference value in-between may have a value selected from a set {−4, −3, −2, −1, 0, 1, 2, 3} and can be represented as 3 bits.

The mode 2-0 corresponds to a case that CQI on a UE-selected band is transmitted in the absence of PMI transmission. In this case, RI is transmitted only in case of open-loop spatial multiplexing (SM) and a WB CQI represented as 4 bits may be transmitted. A best CQI (best-1) is transmitted on each bandwidth part (BP) and the best-1 CQI may be represented as 4 bits. And, an L-bit indicator indicating the best-1 may be transmitted together. If the RI is greater than 1, a CQI for a 1^(st) codeword can be transmitted.

And, the mode 2-1 corresponds to a case that a single PMI and a CQI on a UE-selected band are transmitted. In this case, together with RI transmission, 4-bit WB CQI, 3-bit WB spiral differential CQI and 4-bit WB PMI can be transmitted. Additionally, 4-bit best-1 CQI is transmitted on each bandwidth part (BP) and L-bit best-1 indicator can be transmitted together. Additionally, if RI is greater than 1, 3-bit best-1 spatial differential CQI can be transmitted. In 2-codeword transmission, it may indicate a difference value between a best-1 CQI index of codeword 1 and a best-1 CQI index of codeword 2.

For the transmission modes, periodic PUCCH CSI report modes are supported as follows.

1) Transmission mode 1: Modes 1-0 and 2-0

2) Transmission mode 2: Modes 1-0 and 2-0

3) Transmission mode 3: Modes 1-0 and 2-0

4) Transmission mode 4: Modes 1-1 and 2-1

5) Transmission mode 5: Modes 1-1 and 2-1

6) Transmission mode 6: Modes 1-1 and 2-1

7) Transmission mode 7: Modes 1-0 and 2-0

8) Transmission mode 8: Modes 1-1 and 2-1 if a user equipment is set to make a PMI/RI reporting, or Modes 1-0 and 2-0 if a user equipment is set not to make a PMI/RI reporting

9) Transmission mode 9: Modes 1-1 and 2-1 if a user equipment is set to make a PMI/RI reporting and the number of CSI-RS ports is greater than 1, or Modes 1-0 and 2-0 if a user equipment is set not to make a PMI/RI reporting and the number of CSI-RS port(s) is equal to 1.

The periodic PUCCH CSIU reporting mode in each serving cell is set by upper layer signaling. And, the mode 1-1 is set to either submode 1 or submode 2 by an upper layer signaling using a parameter ‘PUCCH_format1-1_CSI_reporting_mode’.

A CQI reporting in a specific subframe of a specific serving cell in a UE-selected SB CQI means a measurement of at least one channel state of a bandwidth part (BP) corresponding to a portion of a bandwidth of a serving cell. An index is given to the bandwidth part in a frequency increasing order starting with a lowest frequency without an increment of a bandwidth.

3. ACK/NACK Transmission Method Using PUCCH

3.1 ACK/NACK Transmission in LTE System

Under the situation that a UE simultaneously transmits a plurality of ACKs/NACKs corresponding to multiple data units received from an eNB, in order to maintain the single-carrier property of ACK/NACK signals and reduce the total ACK/NACK transmission power, ACK/NACK multiplexing method based on PUCCH resource selection can be considered. With ACK/NACK multiplexing, contents of the ACK/NACK signals for multiple data units are identified by the combination of the PUCCH resource used in actual ACK/NACK transmission and the one of QPSK modulation symbols. For example, if it is assumed that one PUCCH resource carries 4 bits and 4 data units can be transmitted in maximum (at this time, assume that HARQ operation for each data unit can be managed by single ACK/NACK bit), the transmission (TX) node can identify the ACK/NACK result based on the transmission position of the PUCCH signal and the bits of the ACK/NACK signal as shown in Table 17 below.

TABLE 17 HARQ-ACK(0), HARQ-ACK(1), HARQ-ACK(2), HARQ-ACK(3) n_(PUCCH) ⁽¹⁾ b(0), b(1) ACK, ACK, ACK, ACK n_(PUCCH,1) ⁽¹⁾ 1, 1 ACK, ACK, ACK, NACK/DTX n_(PUCCH,1) ⁽¹⁾ 1, 0 NACK/DTX, NACK/DTX, NACK, DTX n_(PUCCH,2) ⁽¹⁾ 1, 1 ACK, ACK, NACK/DTX, ACK n_(PUCCH,1) ⁽¹⁾ 1, 0 NACK, DTX, DTX, DTX n_(PUCCH,0) ⁽¹⁾ 1, 0 ACK, ACK, NACK/DTX, NACK/DTX n_(PUCCH,1) ⁽¹⁾ 1, 0 ACK, NACK/DTX, ACK, ACK n_(PUCCH,3) ⁽¹⁾ 0, 1 NACK/DTX, NACK/DTX, NACK/DTX, n_(PUCCH,3) ⁽¹⁾ 1, 1 NACK ACK, NACK/DTX, ACK, NACK/DTX n_(PUCCH,2) ⁽¹⁾ 0, 1 ACK, NACK/DTX, NACK/DTX, ACK n_(PUCCH,0) ⁽¹⁾ 0, 1 ACK, NACK/DTX, NACK/DTX, n_(PUCCH,0) ⁽¹⁾ 1, 1 NACK/DTX NACK/DTX, ACK, ACK, ACK n_(PUCCH,3) ⁽¹⁾ 0, 1 NACK/DTX, NACK, DTX, DTX n_(PUCCH,1) ⁽¹⁾ 0, 0 NACK/DTX, ACK, ACK, n_(PUCCH,2) ⁽¹⁾ 1, 0 NACK/DTX NACK/DTX, ACK, NACK/DTX, ACK n_(PUCCH,3) ⁽¹⁾ 1, 0 NACK/DTX, ACK, NACK/DTX, n_(PUCCH,1) ⁽¹⁾ 0, 1 NACK/DTX NACK/DTX, NACK/DTX, ACK, ACK n_(PUCCH,3) ⁽¹⁾ 0, 1 NACK/DTX, NACK/DTX, ACK, n_(PUCCH,2) ⁽¹⁾ 0, 0 NACK/DTX NACK/DTX, NACK/DTX, NACK/DTX, ACK n_(PUCCH,3) ⁽¹⁾ 0, 0 DTX, DTX, DTX, DTX N/A N/A

In Table 17, HARQ-ACK(i) indicates the ACK/NACK result for the data unit i. For example, if a maximum of 4 data units is transmitted, i=0, 1, 2, 3. In Table 17, DTX means that there is no data unit transmitted for corresponding HARQ-ACK(i) or the reception (RX) node doesn't detect the existence of the data unit corresponding to HARQ-ACK(i).

In addition, n_(PUCCH,X) ⁽¹⁾ indicates the PUCCH resource which should be used in actual ACK/NACK transmission, if there are 4 PUCCH resources, a maximum of four PUCCH resources n_(PUCCH,0) ⁽¹⁾, n_(PUCCH,1) ⁽¹⁾, n_(PUCCH,2) ⁽¹⁾, n_(PUCCH,3) ⁽¹⁾ may be allocated to the UE.

In addition, b(0), b(1) indicates two bits carried by the selected PUCCH resource. Modulation symbols which are transmitted through PUCCH resource are decided according to the bits. For example, if the RX node receives and decodes 4 data units successfully, the RX node should transmit two bits, (1, 1), using PUCCH resource n_(PUCCH,1) ⁽¹⁾. For another example, if the RX node receives 4 data units and fails in decoding the first and the third data units (corresponding to HARQ-ACK(0) and HARQ-ACK(2)), the RX node should transmit (1, 0) using n_(PUCCH,3) ⁽¹⁾.

By linking the actual ACK/NACK contents with the combination of PUCCH resource selection and the actual bit contents in the transmitted PUCCH resource in this way, ACK/NACK transmission using a single PUCCH resource for multiple data units is possible.

In ACK/NACK multiplexing method (see Table 17), basically, NACK and DTX are coupled as NACK/DTX if at least one ACK exists for all data units. This is because combinations of PUCCH resources and QPSK symbols are insufficient to cover all ACK, NACK and DTX hypotheses. On the other hand, for the case that no ACK exists for all data units (in other words, NACK or DTX only exists for all data units), single NACK decoupled with DTX is defined one as HARQ-ACK(i). In this case, PUCCH resource linked to the data unit corresponding to single NACK can be also reserved to transmit the signal of multiple ACKs/NACKs.

3.2 ACK/NACK Transmission in LTE-A System

In an LTE-A system (e.g., Rel-10, 11, 12, etc.), transmission of a plurality of ACK/NACK signals for a plurality of PDSCH signals, which is transmitted via a plurality of DL CCs, via a specific UL CC is considered. Unlike ACK/NACK transmission using PUCCH format 1a/1b of an LTE system, a plurality of ACK/NACK signals may be subjected to channel coding (e.g., Reed-Muller coding, Tail-biting convolutional coding, etc.) and then a plurality of ACK/NACK information/signals may be transmitted using PUCCH format 2 or a new PUCCH format (e.g., an E-PUCCH format) modified based on block spreading.

FIG. 17 shows an example of a new PUCCH format based on block spreading.

A block spreading scheme refers to a method for performing modulation using an SC-FDMA scheme unlike PUCCH format series 1 or 2 in an LTE system. The block spreading scheme refers to a scheme for time-domain spreading and transmitting a symbol sequence using an orthogonal cover code (OCC) as shown in FIG. 17. That is, the symbol sequence is spread using the OCC to multiplex control signals of several UEs in the same RB.

In the above-described PUCCH format 2, one symbol sequence is transmitted over the time domain and UE multiplexing is performed using cyclic shift (CCS) of a CAZAC sequence. However, in the new PUCCH format based on block spreading, one symbol sequence is transmitted over the frequency domain and UE multiplexing is performed using time-domain spreading based on an OCC.

For example, as shown in FIG. 17, one symbol sequence may be generated as five SC-FDMA symbols by an OCC of length-5 (that is, SF=5). Although a total of 2 RS symbols is used during one slot in FIG. 17, various methods using three RS symbols and using an OCC of SF=4 may be used. At this time, the RS symbols may be generated from a CAZAC sequence having specific cyclic shift and may be transmitted in the form in which a specific OCC is applied (multiplied by) to a plurality of RS symbols of the time domain.

In the embodiments of the present invention, for convenience of description, a multi-ACK/NACK transmission scheme based on channel coding using PUCCH format 2 or a new PUCCH format (e.g., an E-PUCCH format) is defined as a “multi-bit ACK/NACK coding transmission method”.

The multi-bit ACK/NACK coding method refers to a method for transmitting ACK/NACK code blocks generated by channel-coding ACK/NACK or DTX information (meaning that the PDCCH is not received/detected) for PDSCH signals transmitted on a plurality of DL CCs.

For example, when the UE operates on a certain DL CC in an SU-MIMO mode and receives two codewords (CW), the UE may have a maximum of five feedback states including a total of four feedback states of each CW, such as ACK/ACK, ACK/NACK, NACK/ACK and NACK/NACK, and DTX. When the UE receives a single CW, the UE may have a maximum of three states including ACK, NACK and/or DTX. When NACK and DTX are equally processed, the UE may have a total of two states such as ACK and NACK/DTX.

Accordingly, when the UE aggregates a maximum of five DL CCs and the UE operates on all DL CCs in an SU-MIMO mode, the UE may have a maximum of 55 transmittable feedback states. At this time, the size of ACK/NACK payload representing the 55 feedback states may be a total of 12 bits. If DTX and NACK are equally processed, the number of feedback states becomes 45 and the size of the ACK/NACK payload representing the feedback states is a total of 10 bits.

In an ACK/NACK multiplexing (that is, ACK/NACK selection) method applied to an LTE TDD system, fundamentally, an implicit ACK/NACK selection method in which an implicit PUCCH resource corresponding to a PDCCH scheduling each PDSCH (that is, linked to a smallest CCE index) is used for ACK/NACK transmission in order to secure a PUCCH resource of each UE.

In an LTE-A FDD system, transmission of a plurality of ACK/NACK signals for a plurality of PDSCH signals transmitted via a plurality of DL CCs via one UE-specific UL CC is considered. “ACK/NACK selection” methods using an implicit PUCCH resource linked to a PDCCH scheduling some or all DL CCs (that is, linked to a smallest CCE index nCCE or linked to nCCE and nCCE+1) or a combination of an implicit PUCCH and an explicit PUCCH resource pre-allocated to each UE via RRC signaling are considered.

Even in an LTE-A TDD system, aggregation of a plurality of CCs is considered. For example, when a plurality of CCs is aggregated, UE transmitting a plurality of ACK/NACK information/signals for a plurality of PDSCH signals transmitted via a plurality of DL subframes and a plurality of CCs via a specific CC (that is, A/N CC) in UL subframes corresponding to the plurality of DL subframes in which the PDSCH signals are transmitted is considered.

At this time, unlike LTE-A FDD, a method (that is, full ACK/NACK) for transmitting a plurality of ACK/NACK signals corresponding to a maximum number of CWs, which may be transmitted via all CCs allocated to the UE, for a plurality of DL subframes may be considered or a method (that is, bundled ACK/NACK) for applying ACK/NACK bundling to a CW, CC and/or a subframe region, reducing the number of transmitted ACKs/NACKs and performing transmission may be considered.

At this time, CW bundling means that ACK/NACK bundling for CW per CC is applied to each DL subframe and CC bundling means that ACK/NACK bundling for all or some CCs is applied to each DL subframe. In addition, subframe bundling means that ACK/NACK bundling for all or some DL subframes is applied to each CC.

As the subframe bundling method, an ACK counter method indicating a total number of ACKs (or the number of some ACKs) per CC for all PDSCH signals or DL grant PDCCHs received on each DL CC may be considered. At this time, the multi-bit ACK/NACK coding scheme or the ACK/NACK transmission scheme based on the ACK/NACK selection method may be configurably applied according to the size of the ACK/NACK payload per UE, that is, the size of the ACK/NACK payload for transmission of full or bundled ACK/NACK configured per UE.

3.3 Physical Uplink Control Channel Transmission and Reception Process

In a mobile communication system, one eNB transmits and receives data to and from a plurality of UEs via a wireless channel environment in one cell/sector. In a system operating using multiple carriers or the like, the eNB receives packet traffic from a wired Internet network and transmits the received packet traffic to each UE using a predetermined communication scheme. At this time, downlink scheduling is how the eNB determines when data is transmitted to which UE using which frequency domain. In addition, the eNB receives and demodulates data from the UE using a predetermined communication scheme and transmits packet traffic over a wired Internet network. Uplink scheduling is how the eNB determines when to enable which UE to transmit uplink data using which frequency domain. In general, a UE having a good channel state may transmit and receive data using more time and frequency resources.

In a system operating using multiple carriers or the like, resources may be roughly divided into a time domain and a frequency domain. The resources may be defined as resource blocks, which includes N subcarriers and M subframes or predetermined time units. At this time, N and M may be 1. FIG. 18 is a diagram showing an example of configuring a resource block in time-frequency units.

In FIG. 18, one rectangle means one resource block and one resource block has several subcarriers on one axis and has a predetermined time unit (e.g., slots or subframes) on the other axis.

In downlink, an eNB schedules one or more resource blocks to a UE selected according to a determined scheduling rule and transmits data using resource blocks allocated to the UE. In uplink, the eNB schedules one or more resource blocks to a UE selected according to a predetermined scheduling rule and a UE transmits data in uplink using the allocated resources.

An error control method performed when a (sub)frame, in which data is transmitted and received, is lost or damaged after transmitting and receiving data after scheduling includes an automatic repeat request (ARQ) method and a hybrid ARQ (HARQ) method.

In the ARQ method, fundamentally, a transmitter waits for an acknowledgement (ACK) message after transmitting one (sub)frame and a receiver sends the ACK only upon receiving the sub(frame). When an error occurs in the (sub)frame, a negative ACK (NAK) message is sent and information on a reception frame, in which an error occurs, is removed from a receiver buffer. The transmitter transmits a subsequent (sub)frame upon receiving the ACK message but retransmits the (sub)frame upon receiving the NAK message. Unlike the ARQ method, in the HARQ method, when the received frame cannot be demodulated, the receiver transmits the NAK message to the transmitter, but the received frame is stored in a buffer during a predetermined time and is combined with a retransmitted frame, thereby increasing a reception success rate.

Recently, a HARQ method more efficient than the ARQ method is widely used. The HARQ method may be divided into various methods. For example, the HARQ method may be divided into a synchronous HARQ method and an asynchronous HARQ method according to retransmission timing and into a channel-adaptive HARQ method and a channel-non-adaptive HARQ method depending on whether the amount of resources used for retransmission is influenced by a channel state.

The synchronous HARQ method refers to a method of performing subsequent retransmission at timing determined by a system when initial transmission fails. For example, if it is assumed that retransmission is performed every four time units after initial transmission fails, retransmission timing is predetermined between the eNB and the UE and is not signaled. However, when the data transmission side receives a NAK message, the frame is retransmitted every four time units until an ACK message is received.

Meanwhile, the asynchronous HARQ method may be performed by newly scheduling retransmission timing or via additional signaling. The retransmission timing of the previously failed frame may be changed by several factors such as channel state.

The channel-non-adaptive HARQ method refers to a method of using scheduling information (e.g., the modulation method of the frame, the number of used resource blocks, adaptive modulation and coding (AMC), etc.), which is set upon initial transmission, upon retransmission. In contrast, the channel-adaptive HARQ method refers to a method of changing such scheduling information according to the channel state.

For example, in the channel-non-adaptive HARQ method, a transmission side transmits data using six resource blocks upon initial transmission and retransmits data using six resource blocks upon retransmission. In contrast, in the channel-adaptive HARQ method, initial transmission is performed using six resource blocks and retransmission is performed using greater or less than six resource blocks according to the channel state.

Although there are four HARQ methods, the asynchronous and channel-adaptive HARQ method and the synchronous and channel-non-adaptive HARQ method are mainly used. The asynchronous and channel-adaptive HARQ method may maximize retransmission efficiency by adaptively changing the retransmission timing and the amount of used resources according to the channel state but may increase overhead. Accordingly, the asynchronous and channel-adaptive HARQ method is not generally considered for uplink. In contrast, the synchronous and channel-non-adaptive HARQ method may not cause overhead because retransmission timing and resource allocation are predetermined in the system, but has very low retransmission efficiency in a considerably changed channel state.

To this end, in the current 3GPP LTE/LTE-A system, the asynchronous HARQ method is used in downlink and the synchronous HARQ method is used in uplink.

FIG. 19 is a diagram showing an example of a resource allocation and retransmission method of an asynchronous HARQ method.

When an eNB transmits scheduling information in downlink, receives ACK/NAK information from a UE, and transmits next data, time delay occurs as shown in FIG. 19. This is channel propagation delay and delay occurring due to a time required for data decoding and data encoding.

A method of performing transmission using an independent HARQ process for data transmission without a gap during a delay period is being used. For example, if a shortest period from first data transmission to next data transmission is 7 subframes, data may be transmitted without a gap by setting 7 independent HARQ processes. In an LTE/LTE-A system, a maximum of eight HARQ processes may be allocated to one UE in non-MIMO.

4. Carrier Sensing Method in LTE-U System

4.1 LTE-U System Configuration

Hereinafter, methods for transmitting and receiving data in a carrier aggregation environment of an LTE-A band corresponding to a licensed band and an unlicensed band will be described. In the embodiments of the present invention, an LTE-U system means an LTE system that supports such a CA status of a licensed band and an unlicensed band. A WiFi band or Bluetooth (BT) band may be used as the unlicensed band.

FIG. 20 illustrates an example of a CA environment supported in an LTE-U system.

Hereinafter, for convenience of description, it is assumed that a UE is configured to perform wireless communication in each of a licensed band and an unlicensed band by using two component carriers (CCs). The methods which will be described hereinafter may be applied to even a case where three or more CCs are configured for a UE.

In the embodiments of the present invention, it is assumed that a carrier of the licensed band may be a primary CC (PCC or Pcell), and a carrier of the unlicensed band may be a secondary CC (SCC or Scell). However, the embodiments of the present invention may be applied to even a case where a plurality of licensed bands and a plurality of unlicensed bands are used in a carrier aggregation method. Also, the methods suggested in the present invention may be applied to even a 3GPP LTE system and another system.

In FIG. 20, one eNB supports both a licensed band and an unlicensed band. That is, the UE may transmit and receive control information and data through the PCC which is a licensed band, and may also transmit and receive control information and data through the SCC which is an unlicensed band. However, the status shown in FIG. 20 is only example, and the embodiments of the present invention may be applied to even a CA environment that one UE accesses a plurality of eNBs.

For example, the UE may configure a macro eNB (M-eNB) and a Pcell, and may configure a small eNB (S-eNB) and an Scell. At this time, the macro eNB and the small eNB may be connected with each other through a backhaul network.

In the embodiments of the present invention, the unlicensed band may be operated in a contention-based random access method. At this time, the eNB that supports the unlicensed band may perform a carrier sensing (CS) procedure prior to data transmission and reception. The CS procedure determines whether a corresponding band is reserved by another entity.

For example, the eNB of the Scell checks whether a current channel is busy or idle. If it is determined that the corresponding band is idle state, the eNB may transmit a scheduling grant to the UE to allocate a resource through (E)PDCCH of the Pcell in case of a cross carrier scheduling mode and through PDCCH of the Scell in case of a self-scheduling mode, and may try data transmission and reception.

At this time, the eNB may configure a transmission opportunity (TxOP) duration comprised of N consecutive subframes. In this case, a value of N and a use of the N subframes may previously be notified from the eNB to the UE through higher layer signaling through the Pcell or through a physical control channel or physical data channel

4. 2 TxOP Duration

An eNB may transmit and receive data to and from one UE for a TxOP duration, and may configure a TxOP duration comprised of N consecutive subframes for each of a plurality of UEs and transmit and receive data in accordance with TDM or FDM scheme. At this time, the eNB may transmit and receive data through a Pcell which is a licensed band and a Scell which is an unlicensed band for the TxOP duration.

However, if the eNB transmits data in accordance with a subframe boundary of an LTE-A system corresponding to a licensed band, a timing gap may exist between an idle determination timing of the Scell which is an unlicensed band and an actual data transmission timing. Particularly, since the Scell should be used as an unlicensed band, which cannot be used exclusively by a corresponding eNB and a corresponding UE, through CS based contention, another system may try information transmission for the timing gap.

Therefore, the eNB is able to transmit a reservation signal from the Scell to prevent another system from trying information transmission for the timing gap. In this case, the reservation signal means a kind of “dummy information” or “a copy of a part of PDSCH” transmitted to reserve a corresponding resource region of the Scell as a resource of the eNB. The reservation signal may be transmitted for the timing gap (i.e., from the idle determination timing of the Scell to the actual transmission timing).

4.3 Method of Configuring CS Unit for Performing Carrier Sensing (CS)

A unit of a section in which CS is performed may correspond to one OFDM symbol unit based on LTE/LTE-A system. If a current channel state of an S-cell is busy or idle, it indicates that a channel state for one OFDM symbol period is busy or idle. In particular, a result of performing CS performed by a base station in the S-cell corresponds to a representative value of a channel state of the S-cell during one OFDM symbol period.

In the following, various methods of performing CS and a CS unit in consideration of Wi-Fi system as an example of an unlicensed band are proposed.

4.3.1.1 OFDM Symbol Unit

A base station can determine a channel state of an S-cell based on energy amount of a signal measured for one OFDM symbol period. In this case, if the energy amount of the measured signal is greater than a value configured in advance in a system or a threshold value set through X2 interface, the base station can determine it as a busy state. Otherwise, the base station can determine it as an idle state.

4.3.2 Half OFDM Symbol Unit

FIG. 21 is a diagram for explaining methods of performing CS in an S-cell.

In LTE/LTE-A system corresponding to a licensed band, a minimum unit for CS is 1 symbol. FIG. 21 shows one OFDM symbol period. A slot time for performing CS of Wi-Fi system operating in an unlicensed band is about 9 μsec. In particular, referring to FIG. 21, since 1 OFDM symbol of LTE-A system is about 66.67 μsec, Wi-Fi system can transmit and receive data for a plurality of slots in one OFDM symbol period of the LTE-A system.

Referring to FIG. 21, since WiFi transmission is finished, a channel state of an OFDM symbol should be determined as an idle state in CASE1. On the contrary, since WiFi transmission is started, the channel state of the OFDM symbol should be determined as a busy state in CASE2. Yet, as mentioned earlier in the section 4.3.1, if a base station performs CS, it is unable to distinguish the CASE1 from the CASE2 in FIG. 21.

Hence, the present invention proposes to configure a half OFDM symbol by dividing one OFDM symbol into two half symbols. In particular, the base station can measure energy amount of a radio signal transmitted in each half OFDM symbol period.

For example, the amount of energy measured for an OFDM symbol period corresponding to the front half can be defined as E(1) and the amount of energy measured for an OFDM symbol period corresponding to the other half can be defined as E(2). In case of performing CS, the base station can determine a channel state based on the E(1) and the E(2) values.

In this case, the base station compares a specific threshold value configured by higher layer signaling (e.g., MAC or RRC signal) or physical layer signaling (e.g., (E)PDCCH, etc.) with the E(1) and the E(2) to perform CS in a manner of distinguishing the CASE1 from the CASE2 of FIG. 21. In particular, in case of the CASE1, it may be able to induce a CS result of an idle state. In case of the CASE2, it may be able to induce a CS result of a busy state. Or, although the E(1) is greater than the specific threshold value, if the E(2) is less than the specific threshold value, the base station can determine it as the idle state.

However, although the base station determines a channel state by measuring the energy amount of a reception signal in a half OFDM symbol unit, if it corresponds to CASE3 of FIG. 21, an idle state is determined only.

4.3.3 1/N OFDM Symbol Unit

FIG. 22 is a diagram for explaining a case that a unit in which CS is performed corresponds to 1/N OFDM symbol unit.

In the embodiment of the present invention, a case that a unit in which CS is performed corresponds to 1/N OFDM symbol unit is explained. In order to solve a problem corresponding to the CASE3 of FIG. 21, it is important to determine whether a WiFi signal exists at the end of an OFDM symbol. In order to reflect this, as shown in FIG. 22, if an OFDM symbol is divided into N sections and it is determined as an S-cell is in an idle state for the last n number of contiguous sections, the OFDM symbol can be determined as idle.

For example, referring to FIG. 22, the base station can determine a channel state of an OFDM symbol on the basis of the energy amount of the last section N only. In particular, if E(N) corresponding to the energy amount of the last section N is less than a specific threshold value, it can be determined as an idle state or else a busy state. In this case, a threshold value or information on the number (e.g., n value) of sections for performing CS can be configured via a value predetermined in a system or X2 interface.

4.3.4 Generalized 1/N OFDM Symbol Unit

In the following, a case of generalizing the CS unit mentioned earlier in the sections 4.3.1 to 4.3.3 is explained.

A base station can determine an idle state or a busy state of a channel based on energy values E(1), E(2), E(N−1) and E(N) measured in the N number of sections. For example, the base station calculates the sum of energy values to which a weight is applied in a form of equation 4 and compares E_(Total) value with a specific threshold value. If a channel state of an S-cell is greater than the threshold value, the channel state is determined as idle. If the channel state of the S-cell is less than the threshold value, the channel state is determined as busy. In this case, W_(i) value and the specific threshold value may correspond to values predetermined in a system or values determined via X2 interface.

$\begin{matrix} {E_{Total} = {\sum\limits_{i = 1}^{N}\;{w_{i}{E(i)}}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack \end{matrix}$

In equation 4, in case of a CS unit identical to the CS unit mentioned earlier in the section 4.3.1 is configured. In case of a CS unit identical to the CS unit mentioned earlier in the section 4.3.2 is configured. In case of a CS unit identical to the CS unit mentioned earlier in the section 4.3.3 is configured.

And, a CS unit satisfying such a condition as can be configured under an assumption that a reception energy value is more important at the end of one OFDM symbol.

4.3.5 CS Unit Configuration in Consideration of Switching Gap

When a channel is determined as idle by a base station by performing CS, the base station configures a TXOP duration from a next OFDM symbol and may attempt data transmission through an S-cell. In this case, a transmission/reception (Tx/Rx) switching gap occurs. Due to the Tx/Rx switching gap, delay occurs as much as the Tx/Rx switching gap which is delayed from a start point of an OFDM symbol corresponding to a timing point at which the base station actually intends to perform transmission.

In order to solve the aforementioned problem, it may consider the Tx/Rx switching gap at the time of defining the CS unit for performing the CS mentioned earlier in the sections 4.3.1 to 4.3.4. For example, the base station can perform the CS until prior to the Tx/Rx switching gap before the end of an OFDM symbol.

More specifically, when an OFDM symbol size corresponds to t1 μsec and a Tx/Rx switching gap corresponds to t2 μsec, in case of the section 4.3.1, the base station can measure reception energy during t1-t2 time. In case of the section 4.3.4, the base station can perform CS by dividing t1-t2 time into N number of sections.

As a different aspect of the present embodiment, while performing CS in a manner of being identical to the sections 4.3.1 to 4.3.4, the base station can transmit data after puncturing is performed as much as a Tx/Rx switching gap when the base station attempts data transmission in a next OFDM symbol.

The aforementioned sections 4.3.1 to 4.3.5 can also be extensively applied to a case that a P-cell operates in a unit of half OFDM symbol, n number of OFDM symbols, a slot, or a subframe instead of a unit of one OFDM symbol.

In this case, a unit for performing CS selected from the group consisting of one OFDM symbol, n number of OFDM symbols, a slot, and a subframe can be configured via a value predetermined in a system or X2 interface.

4.4 Method of Performing CS in Consideration of Unlicensed Band

In case of a legacy WiFi system operating on 5 GHz band, a base station performs CS for 9 μsec corresponding to WiFi slot time. In the following, methods for a base station to perform CS for 9 μsec in consideration of WiFi system operating on an unlicensed band are explained.

FIG. 23 is a diagram for explaining methods of performing CS in consideration of an unlicensed band.

In embodiment of the present invention, a basic signaling unit of a P-cell operating on a licensed band is 1 OFDM symbol. And, an S-cell operating on an unlicensed band performs CS in a unit of slot time of WiFi system. Similar to the licensed band, the S-cell can perform signaling in a unit of 1 OFDM symbol in a TXOP duration. In this case, 1 OFDM symbol is configured by about 66.67 μsec and a CP length and WiFi slot time is configured by about 9 μsec.

Referring to FIG. 23 (a), assume that the base station performs CS from a first slot time. In this case, the base station can determine that a channel is idle in third WiFi slot time. In order to match OFDM symbol synchronization with a P-cell, if the channel is continuously idle until a start point of a next OFDM symbol, the base station can attempt to transmit data from the next OFDM symbol. If the channel is determined as busy prior to the start of the next OFDM symbol after the third WiFi slot time, the base station can perform CS again in the unit of WiFi slot time from a start point of (N+1)^(th) OFDM symbol.

As shown in FIG. 23(b), if the base station determines that the channel is idle in the third WiFi slot time, the base station may attempt to transmit data by immediately configuring and recognizing a TXOP duration. In FIG. 23(b), T1 indicates a time interval for performing CS and T2 can be defined by a time interval having a size of (1 OFDM symbol length—T1).

According to TS 36.300-related document of LTE/LTE-A standard, it regulates that a UE should be operable in a condition that reception synchronization is not matched as much as maximum 30 μsec between a P-cell and an S-cell. Hence, if the T1 is less than 30 μsec, the S-cell can match synchronization with N^(th) OFDM symbol of the P-cell. Or, if the T2 is less than 30 μsec, the S-cell can match synchronization with (N+1)^(th) OFDM symbol of the P-cell.

However, since LTE OFDM symbol is greater than about 60 μsec, it may have a case that T1>30 μsec and T2>30 μsec are satisfied at the same time. In this case, the S-cell may fail to match synchronization with the P-cell. In order to solve the problem, a maximum permissible range of Tx/Rx synchronization mismatch between the P-cell and the S-cell can be configured by a value greater than 30 μsec. For example, the base station can configure the maximum permissible range of Tx/Rx synchronization mismatch between the P-cell and the S-cell by a value equal to or greater than (66.67 μsec+CP length)/2.

4.5 Method of Performing CS in Consideration of Channel State

In WiFi system, an integer is randomly selected between a contention window size and ‘0’ and the selected integer is defined as a backoff counter value. In this case, whenever a channel is determined as idle after CS is performed, a base station decreases the backoff counter value by 1. When the backoff counter value becomes 0, the base station attempts data transmission at last. In particular, the backoff counter value corresponds to the number of performing CS in every CS unit used for the base station to configure a TXOP duration.

If the base station fails to receive ACK in response to data transmitted by the base station, the base station determines it as transmission has failed due to a collision between the transmitted data and a different WiFi station. In this case, in order to reduce a collision probability, the base station doubles a contention window size. The base station selects a random integer again between the doubled window size and ‘0’ and the selected random integer is defined as a backoff counter value. The base station performs CS until the backoff counter value becomes 0. Subsequently, the base station may attempt data transmission again.

Similar to the WiFi system, when a contention window size and a backoff counter value are defined to perform CS in LTE-U system, it is preferable for the base station to adaptively change the contention window size according to success, collision, and/or channel error of transmitted data (hereinafter, success/collision/channel error).

FIG. 24 is a flowchart for explaining a method of performing CS in consideration of a channel state.

If it is necessary for an eNB to transmit data via an S-cell operating on an unlicensed band, the eNB performs CS to check whether the S-cell is in an idle state or a busy state. In this case, the eNB can perform the CS according to the CS unit mentioned earlier in the section 4.3 [S2410].

If the eNB performs the CS and determines that a channel is in the idle state, the eNB determines whether or not a backoff counter value randomly selected from a predetermined contention window size corresponds to ‘0’ [S2420].

The eNB repeatedly performs the steps S2410 and S2420 until the backoff counter value becomes ‘0’ in the step S2420. If the backoff counter value becomes ‘0’, the eNB transmits data to a UE in a next OFDM symbol or a subframe configured as a TXOP duration [S2430].

It is necessary for the UE to identify whether or not data is successfully received, whether or not data reception is failed due to collision, or whether or not data reception is failed due to a channel error. For example, when data reception is failed, if it is determined as reception strength of a data signal is weak, the UE determines it as a channel error. On the other hand, if it is determined as interference amount is considerably big, the UE determines it as a collision. More specifically, if an RSRP value is less than a specific threshold value, the UE determines it as a reception failure due to a channel error. On the other hand, if the RSRP value is greater than the specific threshold value and an RSRQ value is considerably small, the UE determines it as a reception failure due to a collision [S2440].

The UE can feedback information on success/collision/channel error of a received data, i.e., a reception state of data, to the eNB [S2450].

The UE may use a HARQ ACK/NACK signal as an example of the feedback. For example, in the step S2450, if the UE successfully receives data, the UE transmits an ACK signal to the eNB. If the UE determines a channel error, the UE transmits an NACK signal to the eNB. If the UE determines a collision, the UE can be configured to not to transmit both the ACK and the NACK signal. Or, if the UE determines a collision, the UE can be configured to transmit the NACK signal. If the UE determines a channel error, the UE can be configured to not to transmit both the ACK and the NACK signal.

The step S2450 can be performed via the P-cell. Or, if the P-cell and the S-cell are configured by a self-carrier scheduling scheme, the UE can transmit the ACK/NACK signal via the S-cell. The schemes for transmitting the ACK/NACK signal can be referred to the ACK/NACK transmission method mentioned earlier in the section 3.

Referring back to FIG. 24, after the feedback on data success/collision/channel error is received from each UE via such a method as HARQ ACK/NACK, the eNB can adaptively reconfigure the contention window size based on the feedback [S2460].

For example, if the eNB receives an ACK signal in the step S2460, the eNB determines it as data is successfully transmitted. Hence, the eNB can initialize a value of the contention window size by a minimum value, reduce the value as much as n (>0), or reduce the value m (>1) times. If it is determined as a collision occurs, the eNB can increase the value of the contention window size as much as n′ (>0) or increase the value m′ (>1) times. And, if it is determined as data transmission is failed due to a channel error, the eNB may change an MCS level for transmitting data without changing the value of the contention window size to attempt stable transmission.

The eNB reconfigures a backoff counter within the contention window size reconfigured in the step S2460 and may be then able to perform CS until the backoff counter value becomes ‘0’ [S2470].

If the backoff counter value becomes ‘0’ in the step S2470, the eNB can transmit data again.

The steps S2460 and S2470 can be optionally performed. If the eNB receives an ACK signal in the step S2450, the eNB can continuously transmit data to the UE within a TXOP duration.

The eNB can perform CS again from the step S2410 after the TXOP duration is terminated and before a next TXOP duration starts.

4.6 CS Unit Configuration

According to EU regulation for a licensed band on 5 GHz (c.f., 3GPP RP-140808), it is necessary to perform CCA (clear channel assessment) for at least minimum 20 μsec on a band except 5725 MHz to 5875 MHz to evaluate whether or not a radio resource is available. In order to reflect the regulation, as mentioned earlier in the section 4.3, when a unit of a section in which CS is performed is divided into N number of sections, it may be able to configure each section to be greater than 20 μsec. Or, it may be able to configure a size of the last n number of sections mentioned earlier in the section 4.3.3 to be greater than 20 μsec.

And, as mentioned earlier in the section 4.4, when CS is performed in a unit of WiFi slot time, a unit of k number of WiFi slot times can be configured as a basic unit for performing the CS. For example, if k corresponds to 3, the eNB performs the CS in a manner of configuring 27 μsec section as a CS unit.

Or, the eNB can configure the minimum value of the backoff counter value by an integer c (>0) rather than ‘0’. For example, if the c corresponds to 4, each eNB selects a random integer between the contention window size and 4 and may be then able to perform CS in a unit of the number of WiFi slot times as many as the selected value. In this case, the k and the c can be configured via a value predetermined in a system or X2 interface.

5. Apparatuses

Apparatuses illustrated in FIG. 25 are means that can implement the methods described before with reference to FIGS. 1 to 24.

A UE may act as a transmission end on a UL and as a reception end on a DL. An eNB may act as a reception end on a UL and as a transmission end on a DL.

That is, each of the UE and the eNB may include a Transmitter (Tx) 2540 or 2550 and a Receiver (Rx) 2560 or 2570, for controlling transmission and reception of information, data, and/or messages, and an antenna 2500 or 2510 for transmitting and receiving information, data, and/or messages.

Each of the UE and the eNB may further include a processor 2520 or 2530 for implementing the afore-described embodiments of the present disclosure and a memory 2580 or 2590 for temporarily or permanently storing operations of the processor 2520 or 2530.

The embodiments of the present invention can be performed using configuration components and functions of the aforementioned UE and the eNB. For example, a processor of the eNB can configure a CS unit for performing CS by combining the methods disclosed in the aforementioned sections 1 to 4. Of course, the CS unit may correspond to a predetermined value on a system. The eNB performs CS based on the CS unit to determine whether an S-cell is in an idle state or a busy state and may be able to transmit and receive data. And, the eNB can transmit and receive ACK/NACK signal in response to the transmitted and received data to adaptively control the CS unit.

The Tx and Rx modules of the UE and the eNB may perform a packet modulation/demodulation function for data transmission, a high-speed packet channel coding function, OFDMA packet scheduling, TDD packet scheduling, and/or channelization. Each of the UE and the eNB of FIG. 25 may further include a low-power Radio Frequency (RF)/Intermediate Frequency (IF) module.

Meanwhile, the UE may be any of a Personal Digital Assistant (PDA), a cellular phone, a Personal Communication Service (PCS) phone, a Global System for Mobile (GSM) phone, a Wideband Code Division Multiple Access (WCDMA) phone, a Mobile Broadband System (MB S) phone, a hand-held PC, a laptop PC, a smart phone, a Multi Mode-Multi Band (MM-MB) terminal, etc.

The smart phone is a terminal taking the advantages of both a mobile phone and a PDA. It incorporates the functions of a PDA, that is, scheduling and data communications such as fax transmission and reception and Internet connection into a mobile phone. The MB-MM terminal refers to a terminal which has a multi-modem chip built therein and which can operate in any of a mobile Internet system and other mobile communication systems (e.g. CDMA 2000, WCDMA, etc.).

Embodiments of the present disclosure may be achieved by various means, for example, hardware, firmware, software, or a combination thereof.

In a hardware configuration, the methods according to exemplary embodiments of the present disclosure may be achieved by one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, etc.

In a firmware or software configuration, the methods according to the embodiments of the present disclosure may be implemented in the form of a module, a procedure, a function, etc. performing the above-described functions or operations. A software code may be stored in the memory 2580 or 2590 and executed by the processor 2520 or 2530. The memory is located at the interior or exterior of the processor and may transmit and receive data to and from the processor via various known means.

Those skilled in the art will appreciate that the present disclosure may be carried out in other specific ways than those set forth herein without departing from the spirit and essential characteristics of the present disclosure. The above embodiments are therefore to be construed in all aspects as illustrative and not restrictive. The scope of the invention should be determined by the appended claims and their legal equivalents, not by the above description, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein. It is obvious to those skilled in the art that claims that are not explicitly cited in each other in the appended claims may be presented in combination as an embodiment of the present disclosure or included as a new claim by a subsequent amendment after the application is filed.

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to various wireless access systems including a 3GPP system, a 3GPP2 system, and/or an IEEE 802.xx system. Besides these wireless access systems, the embodiments of the present disclosure are applicable to all technical fields in which the wireless access systems find their applications. 

What is claimed is:
 1. A method for performing carrier sensing by a base station in a wireless access system supporting an unlicensed band, the method comprising: selecting a backoff counter value for performing a carrier sensing procedure, based on a contention window size; decreasing the backoff counter value based on a determination that the unlicensed band is an idle state while performing the carrier sensing procedure in the unlicensed band; transmitting downlink data based on the backoff counter value being equal to 0; obtaining a hybrid automatic repeat request acknowledgement (HARQ-ACK) feedback for the downlink data; and adjusting the contention window size based on the HARQ-ACK feedback, wherein the adjusting of the contention window size comprises: initializing the contention window size or decreasing the contention window size when the HARQ-ACK feedback includes an acknowledgement (ACK) signal for the downlink data; increasing the contention window size when the HARQ-ACK feedback includes a negative acknowledgement (NACK) signal for the downlink data; and maintaining the contention window size and changing a modulation and coding scheme (MCS) level when the HARQ-ACK feedback includes neither the ACK signal nor the NACK signal.
 2. The method of claim 1, wherein the downlink data is transmitted via the unlicensed band, and wherein the HARQ-ACK feedback is received via a licensed band.
 3. The method of claim 1, wherein the carrier sensing procedure is performed in a carrier sensing unit interval.
 4. The method of claim 3, wherein the carrier sensing unit corresponds to 1 orthogonal frequency division multiplexing (OFDM) symbol unit, a half OFDM symbol unit, or a 1/N OFDM symbol unit, and wherein N corresponds to a natural number.
 5. The method of claim 4, wherein the unlicensed band is determined to be in the idle state when it is determined that the last n number of consecutive sections in the N number of 1/N OFDM symbol units are idle.
 6. A base station performing carrier sensing in a wireless access system supporting an unlicensed band, the base station comprising: a transmitter; a receiver; and a processor, wherein the processor is configured to: select a backoff counter value for performing a carrier sensing procedure, based on a contention window size; decrease the backoff counter value based on a determination that the unlicensed band is an idle state while performing the carrier sensing procedure in the unlicensed band; control the transmitter to transmit downlink data based on the backoff counter value being equal to 0; obtain a hybrid automatic repeat request acknowledgement (HARQ-ACK) feedback for the downlink data from; and adjust the contention window size based on the plurality of HARQ-ACK feedback, wherein the processor is further configured to: initialize the contention window size or decreasing the contention window size when the HARQ-ACK feedback includes an acknowledgement (ACK) signal for the downlink data; increase the contention window size when the HARQ-ACK feedback includes a negative acknowledgement (NACK) signal for the downlink data; and maintain the contention window size and change a modulation and coding scheme (MCS) level when the HARQ-ACK feedback includes neither the ACK signal nor the NACK signal.
 7. The base station of claim 6, wherein the downlink data is transmitted via the unlicensed band, and wherein the HARQ-ACK feedback is received via a licensed band.
 8. The base station of claim 6, wherein the carrier sensing procedure is performed in a carrier sensing unit interval.
 9. The base station of claim 8, wherein the carrier sensing unit corresponds to 1 orthogonal frequency division multiplexing (OFDM) symbol unit, a half OFDM symbol unit, or a 1/N OFDM symbol unit, and wherein N corresponds to a natural number.
 10. The base station of claim 9, wherein the unlicensed band is determined to be in the idle state when it is determined that the last n number of consecutive sections in the N number of 1/N OFDM symbol units are idle. 